They are great cars for the money, and very reliable. A colleague of mine bought a 2.2 with over 100k miles for £1500 and it's not skipped a beat in 2 years. Granted, the 4 pot engine doesn't sound the best so definitely get a 3.0 straight 6. But the general reliability is very good.
Get yourself into the Z4 forum for mod ideas and such.
the only weak points were the steering - which can be remedied now. if you feel sticky steering take it to an indy and get him to slacken the steering by 6mm. should fix it otherwise it's £1500 for a repair by fitting a used one or £3500 to fit a new one
and the roof motor. get it relocated into the boot for £80 on the forums.
other mentionable fixes are
VANOS - rebuild, isn't necessary but the cars timing works a lot better
DISA valve - take it apart and if it's still working rebuild it using metal instead of plastic. if it's knackered replace and rebuild the replacement. this is an easy job. even for people with zero skills. i did mine myself.
clean the throttle control thingmajig - not done mine probably should this summer.
rear brake light - fix this yourself every mechanic overtightens the screws causing them to crack. do it yourself and don't fully tighten the screws just enough to secure it and they should last forever.
rear springs - tend to snap/crack will need replacing every 4 years or so. depends on usage though.
the sound and speed for the money is unmatched IMO. steering is now loose on mine due to the fix but it just adds to the experience as its fun turning it more for the smaller movement.