1.- Really important. You are talking about $200-500 CLA charges here.
2.- Depends on what is to be done on the CLA. For a regular CLA, I would go for Youxin Ye (yyyeCamera). For a complete overhaul, would go for Don at DAG Camera. Just my experience though, and I bet everyone will tell u a different story.
3.-Not sure I would recommend ebay for a CLA, fully functional Leica. Everyone has a different concept about CLA so YMMV. If you get a bargain and adding +$200-500 for CLA sound good to you, then go ahead. If you would like to save problems, then locally or probably classified on this forum. Lots of decent people and, most importantly, actual photographers selling here.
4.-I've 2 M4 and had one M4-P. I would say slighltly noticeable on film advance and probably weight. Call me an idiot but M4-P feels a little bit more "cheap" but not actually a problem. Otherwise, hardly noticeable unless you like 28mm. By the way, I sold M4-P because I sometimes use the self timer and I prefer that feature and I seldom use a 28mm (use M6 for that).
I think both of them are great cameras. Just prefered M4 due to the self timer.
Regards
Marcelo