Which equipment to buy with 15-30k budget for nebulaes / deepsky object photography

Yes, your dad needs a lightweight mount, something like new harmonic drive mounts with no need for counterweights.
Camera wise, go with mono for the best result, but consider longer integration and learning / stacking curve. Might be too much complicated for him.

When it comes to telescope though, either go with a small APO refractor like FSQ-85 or consider RASA8. Both will be good and they are very different.

As Andrea pointed out we need more information for an informed recommendation.

Let's be very honest: The capability of your father to carry equipment will not improve over the upcoming years. So maybe something easy and lightweight is the way to go instead of something too ambitious. I'm in the process of deciding on a mount myself and get more and more inclined towards something which already proved that it works well, instead of aiming at the newest and latest. The ZWO AM5 looks promising but the final verdict is not yet out. With your budget I would recommend the RST-135 or RST-135E. 

Another appraoch could be to check whether a fixed installation of a mount on a pier in the backyard is an option in order to avoid lugging the stuff around. Maybe gifting him an observatory for his existing gear is the way to go?

With respect to the other stuff it all boils down to the question how much time and effort your father is intending to put into learning astrophotography if he prefered to do visual astronomy so far. Maybe he's happy restricting himself to EAA?

Clear skies
Wolfgang

The best thing I did in astrophotography was to install a permanent setup in the front yard. It eliminated lugging heavy equipment and setup time is just a few minutes. In my case I purchased a Skyshed Pod ($3,000 Can.) It’s easy to assemble and they ship all over the world. Of course this is all dependent on being able to have the space to do it.  This also widens the choice for equipment as weight is no longer a factor. Also, for deep sky stuff my vote would go to a refractor.

‘’Hank

I don't know what to tell you to buy. But I can tell you I will need to buy some kleenex as I get teary thinking about how happy I would be with that budget.

Some great suggestions above.  

I am not which round birthday it is (but I guess 60 or 70). As someone who sits in between, and I had a son who wished to spend that amount then I would vote for either a permanent observatory or subscriptions to remote observatory.

For that amount, a 10 year subscription to a consortium would generate all the images he could handle….

The fact he sold his EQ6R may indicate he is less interested in doing it himself these days.  When I passed 60 and decided I had enough of lugging heavy kit around (including the EQ6R) I build my own observatory to house it.  

One issue was the weight, but the other issue was setup time.  With a permanent observatory, I probably save myself about 1hour a night minimim in set up and tear down time.

 At 60 or 70, remaining quality time is increasingly important.  Hence subscription or observatory.

For serious work you would need slightly better gear I would say. Your budget is healthy and you are welcome to take a look at my setup and sample images:
https://rouzastro.com/observatory-2/


This is in my backyard you can click on the recent images in the gallery for samples:
https://rouzastro.com/nebulae/



CS
Rouz

Thank you for all of your suggestions. Some really helped me already quite a lot! Thank you so much for this.

My father obviously was overwhelmed. He turned 69 now. The gift is for today`s and also his 70th birthday. We were able to talk about various models, setups etc. .
Also we have been at his local astro dealer and had a great informative session there. Together with all your great inputs, slowly we get closer to a preferred setup.

The setup we are aiming for should still be a portable lightweight setup. 
Unfortunately a permanent setup in the garden as well a mount with wheels is no option for him, given the space he has. He prefers a quick and easy to setup rig.
He is not willing and able to carry heavy equipment around. Maybe in few years for myself?

What do we want to achieve with the setup?
Photography of whatever is possible with a light, portable but advanced setup. Set it up in the garden, set the program and let it capture through the night. He likes galaxies. I like spectacular colorful nebulaes. 
If the setup allows also visual observation, it would be a nice to have as it would make most of his other lightweight manual setups more or less obsolete.
About time he wants to spend, I think the easier, lighter, faster to setup, the more he will use it. I expect he would use it 2-3 times a month, also limited by unfavorable weather. 


Telescope:
As for the scope, a refractor is almost set, I think around a focal length of 400-800mm. Plus additional "x1.6" lens or similar.
An FSQ106 looks quite promising, only issue is that it would be for camera setup only, non visual. Also the leadtime of 6month could be an issue, albeit not a killer criteria. FSQ106 vs FSQ85 opinions?

His dealer also threw in an option for a Borg 107FL which we had a chance to see in person. There is just little but still useful feedback available online.
Super lightweight, Carbon tube, impressive optics, for visual and camera suitable. On the negative side what I read online, maybe issues with true colors? Also some issues with scratches on the aluminum. Price wise 2/3 of a FSQ.

William Optics, super many youtubers are using them, crazy. But then I also read some critical reports with quality issues, also mentioned here in this thread. For this reason, as of now I would just consider it as "starting with" option.

TS-Optics, Astrotech, Askar etc. are not just another chinese products? Worth to consider as in between Williams Optics and TAK?

Mount:
Harmonic drive is almost set. His dealer recommended not to go with a ZWO AM5 mostly due to the steep tracking inaccuracies which can get difficult to get down to a low enough level even with tracking. Rather he recommends something like a CEM40(EQ).
As for the CEM40 I found there are incompatibilities in  between ASIAIR and the CEM40 electronics which requires a USB HUB in between to solve some incompatibilities. A no go for an electronics engineer like me. 
HEM44 is not yet available, but HEM27 seems to have the same compatibility issues.
There are other recommendations here like the products from rainbow astro like RST-135 or RST-135E. Still have to study them now
A big concern for me is how fast a good polar alignment can be achieved, how easy, quick and accurate it can be set.
Plate solving with ASIAIR and an easy to adjust and accurate mount?

Controller:
For ease of use and peace of mind, I still tend towards ASIAIR. But then I did not spend much time yet to check out the other systems yet, albeit I already installed and looked into software like NINA etc. 
His dealer was not fan of ASIAIR and recommended to go more into a windows setup with more software options like Phoenix, NINA, etc.
Thoughts?

Camera: 
So far I tend to go into the direction of mono. Therefore I would compromise on the sensor size and could accept a APS-C sensor size. 
Probably something like ASI2600MM, ZWO 2" Filterwheel and OAG, EAF, ASI120mm guide camera, Astronomik filters. 
Light pollution is an issue also at our place in central Switzerland.

Curious about your further thoughts and inputs, really appreciate and helpful!

Like many on here, I use NINA running on a miniPC running Windows to control everything.  In my case I have the miniPC strapped permanently to one of my tripod legs.  I then control that miniPC using remote desktop over wifi from my main Windows processing desktop inside. I purchased the Mele Quieter 3 miniPC and it works fine.

Nina has a 3 star polar alignment add-in that doesn't need line of site of Polaris.  It works very well and is a very quick polar alignment tool.

Stellina is a great thought for easy + nice cross between AP and visual. Nice idea! 

Very much agreed that under 1200mm or so of focal length, you're not going to have a lot of luck on galaxies. 8" EdgeHD is probably a best bet there, due to weight and focal length. I wouldn't advise that huge of a refractor. 

As for FSQ 106 vs 85, I've spent a lot of time window shopping on them, and the aperture on the 106 is seriously appealing, but I've seen excellent results with the much lighter 85. 

As for AT and TS, I see a lot of people move to them after WO. Better QC from what I can see, and everybody I've seen with them has had good things to say. I would view them as a nice step between WO and Tak. My next scope will probably be an AT, but I'm seriously eyeing those TS 96mm photolines. 

The folks at astronomics quality check the ATs before sending them out. The folks at Telescope Service check the TS scopes. Those are serious draw for me, personally.

Considering your budget, save a part of it for a powerful desktop (anyone tried the mac studio around here?) and a color calibrated screen if he doesn't have one already. We spend so much time stacking and working images…

Just a suggestion: if you can't build an observatory, buy him a wheeley bars or similar rolling tripod.  You can mount a large scope on that and he will be able to wheel it out to the yard.  I use a rolling tripod and can be setup in 5 mins. Batteries are mounted to the tripod so there's literally no setup, just turn them on and go.