I went down this road recently.
I decided that, for my use, I'd get as much fun out of a Z3 as a Z4 and at a much lower cost. And, I liked the looks of the Z3 over the 4.
I talked to people who own or had owned Z3s and pretty much got a "thumbs up" that it was a good car to own.
My criteria were: 6cyl, roll bars, red, one owner, reasonable mileage; not from the snowbelt. After looking at a few, I decided that the 2000 year model would be my floor because of the improved top, altough it was power operated which I had hoped to avoid for simplicity sake.
I tracked Z3s on Autotrader for over a month and I subscribed to Carfax reports. While Carfax isn't bulletproof, it sure helps eliminate a lot of undesirable cars.
I evaluated 107 Z3s. I called about several but actively pursued onlly the one that I purchased. The car had been on Autotrader for over a month with only one crappy picture and brief (and erroneous) description. The owner said that I was the only person who called. It had 49K milesand met my criteria. I had a friend who lived 50 miles from the car go and evaluate it against a two page check list of items that I gave him to look at. He gave me the go-ahead and I made the deal and had the car delivered by the seller to my friend's house where we picked it up. I've been very happy with my Z-3. My only regret is that I waited so long to get off my motorbike and into a Z3.