I was seriously considering buying an '01+ M Coupe (which is the hard top version) but the prices were always astronomical because of the rarity. The Z3 Roadsters are much more plentiful. There were several engines offered for Z3, so power is dependent on engine choice. I think they are all fine except the 1.9 liter motor. The MZ3, of course, will be the more exciting with the S54 engine MZ3 ('01+) being tops.
They are fun cars and there is a large following which means lots of aftermarket parts. My only qualm is the rear suspension and rear subframe. The rear suspension is the old semi-trailing arm E30 suspension (pre 1991 style) and has quite a bit of squat and camber changes on acceleration/braking. The rear subframes also have a problem with tearing out. Maybe not entirely common but when/if it does happen, it is expensive to fix. There are ways to reinforce it though. Be sure to jack up the rear and crawl under there to check for such cracking before buying any of these.