Hi, welcome to the forum. I'd decide whether or not to buy the car on the way it looks, how it drives and runs how clean it is etc. Z3's are pretty solid and don't really suffer from body rot but there are exceptions. You can check MOT history on the DVLA site which will give you an insight into how it's been looked after as all advisories are recorded for everyone to see. If you aren't mechanically minded try to get someone to look at it for you.
There are always cars for sale here on the forum and the best thing about buying one here is that it's owned by an enthusiast. Good luck.