Atmos is doable but then you will get into something with lower output and thats not the direction you want to go if thats about all you want to spend otherwise you will have to go newer and the price increases.
The Prime family can be easily driven with any quality AVR. The above recommendation for the X4000 (or anything in the current 4000 series - 4100, 4200) is very solid.
We prefer Denon and Marantz because they use Audyssey (look for MultEQ XT or XT32 as the type). External power is typically not needed, but if you have a very large room, sit far from the speakers and like to play the system at theater-like levels, then additional clean power (especially on the front stage) could be a good idea. Hard to beat Emotiva in that department - the XPA-3 is super popular for the LCR and then let the AVR power the rest.
If you think external power might be in your future, just make sure the AVR has pre-outs for most of the speaker channels. It's an increasingly rare feature these days, even in mid-priced units.
+2 on the 3 channel amp for the front stage, LCR. I added an XPA-3 and had plenty of juice left in the AVR to power the remaining speakers. Buy the best AVR you can afford, first. An external amp is something you could add at a later time, if you feel the need. Having experience with the Prime series, I think a good AVR might suffice.