117k - 2 previous owners - new engine at 108k just before we bought it 7 years ago - great fun ! My wife says she will divorce me if I sell it - now there's a thought ( only joking dear if you read this ! )
I would say that's seen a lot of love and some metal surgery at some point. I have a friend that restores these and runs them as a fleet of hire campers so I've seen the state they get in when left for a while.
It's a good question though. I was working on the bodywork on my 22 year old French snotter yesterday and noting that the underside is relatively rust free I started to wonder how a z4 will look at that age.
There is a thread on PH with a guy stripping an e46 330i and the underside is pretty crusty. Underseal can hide a multitude of sins until a mechanic pokes it with screwdriver at MOT time.