My little, beat-up 318 that is my every-day driver (sometimes 200 miles or more a day) has over 200,000 miles on it. Doesn't use oil, no smoke, runs like a trooper. The thing about the ones that have been modified is you want to be sure the person who did the mods has a clue. You can get into real trouble when some idiot gets in there and starts messing around.
If you buy one of the higher end 5s used you've got a pretty good shot that they've been well maintained; it's when you get into the lower tiers that you have to wonder about the maintenance. With proper maintenance though, they're pretty much bullet proof. The 4.0 liter motor is the only one that was really ever "iffy." The 6 series can get expensive as far as maintenance. The little 1.8 liter cars are bullet proof. They've been making that motor FOREVER and the bugs are totally out of it. Parts are dirt cheap too. We had to replace the clutch and two front wheel bearing assemblies when we bought the 318 (for $700) and the parts were only a little over $200. I just picked up a set of Recarro seats for the front for $75. Comfy!
Yeah, I'd love to have an M. But I want an M Coupe. My dream car, lol. And an early '70s 911. And a 944Turbo. And a mid '80s Mercedes 5.0 liter ragtop. The unrealistic one though, would be an AMG Mercedes . . . ragtop . . . All straight shift, of course!