Brian Schumacher:
Unfortunately, astrophotography is an expensive hobby, and most of us quickly realize that our list of wants becomes never-ending. If you plan to evolve in this hobby, I recommend focusing on a good go-to mount. The mount is the cornerstone of your gear. To be clear, an average entry level system will cost between 3 to 4 thousand dollars. In the past, I used to promote basic, inexpensive start-ups to our astronomy club, but I have since shied away from giving that advice. Astrobin has a lot of great resources that allow you to drill down on equipment to help you with possible purchase choices. It's a great tool. You have also not indicated what type of astrophotography as deep space is long exposure and planetary/Luner is normally (lucky imaging). There is a difference in the type of gear that is required for each.
My advice is to take your time, do lots of research and have FUN!
Brian brings up a great point about DSO vs Planetary (my mind always goes to DSO)…
If you start with planetary, your Nexstar mount should work (I use a Nexstar 8SE). Add a ZWO ASI585MC camera and you’re ready to go.
Based on your results, you can decide on a Barlow/Powermate, ADC, etc…