What a machine is all I can say. I have a constant smile every time I drive her. There are bad points like any car but believe me, the good out weigh the bad by miles.
For piece of mind I would definitely only go for one that still has comprehensive warranty active.....mines costing €100 a month for that. They are not exactly a reliable car.....I have had €6k worth of warranty work done in the past year., so it has paid for itself over and over again
Next is the dreaded fuel economy, don't even ask, they are complete pigs. I am using mine as a daily commute back and forth to work through town, I'm just about seeing 200km to a €75 tank. but you'll quickly forget about that when you sink the boot.
Probably the most notorious and labour sensitive preventative maintenance that any M5 owner should do IMO is the rod bearings and bolts. A quick google search will return a tonne of info and disaster stories. They were a design flaw from the beginning and ppl have seen failures in engines as low as 20k miles. I have just got this work completed as mine is shy of 60k. There was a good amount of wear found.
I am running Eisenmann race back boxes, Evolve Eventuri CF Intakes and an RPi under drive pulley. The sound and pull is outstanding. My Evolve Alpha N map is due to arrive later this week, which is supposed to transform the car.