Buy BMW Approved Used Car (AUC) to get remainder of 3yr warranty and full main dealer support. There's quite a few well optioned 07 07 cars out there, all with reg nos Y*07*** ..., at the dealers at the moment with prices around £32 - 36k depending on mileage and options. There's not much in the way of problems to look out for. The M3/CSL running gear and the base Z4 are well sorted and proven and any early parts failures will be covered by warranty anyway. The only problem I've had in 35000mls of Z ownership is a broken rear spring - I blame 10 speed bumps a day... sigh! I would ask to see the service record to see if its had warranty work and/or crash damage repairs. The dealer that has done the work can see the details other dealers can only see its been in and where so it's possible to call them for info if necessary. It may not have been to a BMW bodyshop so check panels, gaps inside and out and look for evidence of paint spray. Originally the Z4's have a dark grey internal painted surfaces, engine bay for example, that's hard to replicate.
Buy on condition, scratches, parking knocks etc and take a close look at the tyre shoulders. Any abrasion beyond the normal contact patch will indicate the car has been driven (cornered) hard. Check the mileage to service, add to the total mileage and if its much less than 14000mls it might have been driven hard although it's quite hard and scarey to thrash a Z4M. In fact some would say ignoring the running in limits will produce a free reving engine!
Make sure all the manuals are there, stamped up by a BMW dealer. The Z4M has a 1400mls running in service/
