Mane - I, too, had a desire to convert my coupe from manual cranks to power. This was years ago now, so I sourced front regulator/wiring/switches, etc., setups from a fellow E9coupe member (from a parts car he had). Then I stored the whole bunch in my garage attic for years (I've had my coupe for over 20 years).
A couple of years ago, my manual cranks started failing (felt like the internal gears were slipping, resulting in my front windows sticking half open, or, unable to get up from the fully down positions). Upon diagnosis of the problem (via removal of door panel, removal of entire regulator mechanisms), it turned out that the key pivoting points of the mechanisms were made out of pot metal, had stripped, were integral parts of "assemblies" (riveted, enclosed, collections of gears), thus rendering them not repairable. So, I "hard wired" the window regulator arms in the "up" positions and drove thusly for nearly two years. Meanwhile, I sourced some E28 rear window motors and did some learning on how to install them in our coupes. There are many threads on this process on this forum. Same same for adding relays for the motors.
Replacing manual cranks with electric ones is fairly straight forward, given that you have prepared appropriately. That is, you have the mechanisms, motors, additional switches for the console (spade vs prong types), are prepared to drill holes in your doors and door bulkheads (near the mounts for the door brakes) for the wires required, have considered the merits of adding circuit relays (where and how many), etc. By the way, the actual mechanisms (regulator mounts and scissor bars) are identically mounted in the doors. No drilling required. But, of course, you are left with serious holes in the door panels where the cranks were.
All in all, I am very happy with my end result. But, it was indeed a complicated project. Decisions, decisions. You'll be making many should you travel down this road.