I came across this on MSN. It may be useful if you're considering a Z4M.
http://www.msn.com/en-gb/cars/ownership/used-car-buying-guide-bmw-z4/ar-BBETCLo
Tony.
http://www.msn.com/en-gb/cars/ownership/used-car-buying-guide-bmw-z4/ar-BBETCLo
Tony.