I've done a ton of research here, I have a 136 and 134. Here is what I've learned from Don/DAG.
1) The major shutter related issues are fully under 130x. In the Leica production period they corrected the issue and repaired the existing 128-9 units.
2) Erik is correct. I currently have my 134 getting a replacement frame set as the RF and frames were on their way to total failure. It appears parts here are really limited too, I think I'm getting lucky with another M5 set and not a M4 set....not that it matters more then resale. If it looks squiggly....drippy in there, reconsider.
3) I'd add to look in the battery compartment too. If it's corroded I'd reconsider it.
4) factor in a CLA to whatever this costs. 200 to 400. I prefer the M5 more than any M but the mechanics are complex and if you want to treat it like a MP or an F6 Sherry and Don need to get inside there and do their magic....just to be sure.
Good luck though, either way. If you get bit by the M5 bug it's hard to deal with. I sold a M3, M4, Yashica T4 and Oly XA to support my M5 habit!