Thanks for all the responses so far. I've never come across such a helpful forum. It must be something about the Z4 that brings out the best in people.
The M is out of the question because I don't want to spend more than £6.5k. I fully appreciate the advice regarding trying them all but even that is proving difficult without travelling a long way from Dorset. There is a newer 3.0 at a dealer very near me which has been mentioned on the forum with hardtop and in an unusual dark metallic green colour, but it's nearly double my budget and I don't feel comfortable about taking a test drive when I have no intention of buying. I wouldn't do it to a private seller either.
A couple more questions arise from the comments so far. Is the faceliift version much different than the earlier version? I'm not bothered by the revised lights and the slightly better protected (?) roof motor, but is there more to it than that? Also, is it just an urban myth that prices fall once summer is over? I'm worried that very few will be up for sale shortly because sellers will wait until next spring.
Many thanks for your help.