The 980 is nearly TWICE as fast as an M395X and Radeons CANNOT work with CUDA acceleration or openGL, so you're stuck with OpenCL, and stuck with that GPU for as long as you have the iMac. I'm a huge Mac fan but please don't make this move! If anything get a 980TI ... I'm running that in my Mac Pro and it KILLS with CUDA in after effects.

Michael pretty much summed it up but to add one point: 85mm is not all that different in FOV compared to 50mm. Is it noticeable? Of course, but if you've only got those other two lenses you're going to get a lot more value out of renting something like a 100 or 135, or even 200mm.

To help us help you please tell us how you plan to use the camera:
On documentaries or narratives?
With lights or without? Low light or in bright daylight?
Do you have any lenses already?
Will you be on a tripod or handheld?
What kind of "look" are you trying to achieve?
Etc. Etc.

Pablo, from the way you phrased your question, I'm not sure you're aware that Adobe CC DOES get installed on your computer...

Why not spend the $1K on someone who's willing to teach you while he/she does their job? You'll learn more in a day then scouring the internet for weeks.

Just FYI Joshua, XAVC-S is just a specific version of h.264. I know that 1080p24 from an A7S is:
High Profile Level 4.1
GOP = 6 (keyframe every 6)
2 B-Frames

Pretty certain this stays the same for every frame rate on an A7S(II).