Coupe or Cab; your choice. But, a few questions/comments.....
1) Have you driven one?? 'Big difference from M5 sedan. You need to know you'd be happy with long weekend drives and the commute.
2) 'Dunno what your $$$ budget is, but '02 - '04 model year are best. '05 is good, too; but quite rare and therefore much more $$$.
3) If you haven't already, peruse
www.6speedonline.com . Lot's of good info there; including many anecdotal threads related to reliability/durability. 'Fair to say, these cars are relatively "bullet proof", if maintained properly. CPO or after market extended warranty is "nice to have", but certainly not worth sweating over.
4) If you search, try finding one with X50 pkg option. It was a $19k power option (GT2 turbo's and intercoolers) when new, but little to no premium now in used market. And for Gawd's sake, get a 6-speed manual. Tiptronic autobox is crap.
5) These cars when new and nicely optioned were $150k - $170k. Like all "toys" in this economic climate, prices are down and a pristine, low miles example can be had for $65 - $80k, depending on MY.
6) Just cruising and mostly staying "off boost" will get you 22 - 26mpg. Even "mixed" driving (playing with it's power) will still result in 18 - 20mpg (that's what I average, even though mine is highly modified).
Good luck and happy hunting!!