How do they cope with big miles, say 100k+?
If they get there ok from wha I know they should do another 100k, it's a very strong engine. With age the issues are head gaskets and potentially vanos bolts. So if your buying a high miler do some preventative maintanance:-
1. Check engine compression, if two adjacent cylinders are sub 150psi, get a head gasket kit for about £300 and fit it, takes circa 4hrs. Over 150psi and all fine, a new engine has around 185psi and mine 91k miles on is still on 185psi.
2. Change vanos filter - £5
3. Change vanos bolts, if they go they take engine, not expensive and not too hard to do, if vanos fails for other reasons it can be repaired/refurb for about £800
4. Change oil every 6k
5. They are very sensitive on fuel, run on only 98+
6. ensure maf and filters are clean, if you change reset long term fuel trims to prevent any running lean issues.
7. Manuals are fine, SMG has some issues but again pretty solid.
8. Ensure inspection 2's are not skipped - valve clearances, say every 20k miles.
Do the above and I see no issue for superb reliability.
Facts are 6k can get you a very clean well cared for M3, I know one for sale at 5.5k which is very clean and solid. They are absolute bargains and one seriously capable car in the dry.