Welcome Paul!
I know the feeling of needing that little mid-life reward. (I'm 66) I picked up my '97 2.8 Z3 in November. So far, it's been rock-solid, and even enjoyed a 400 mile day with the local BMW club doing some VERY spirited driving along the Blue Ridge mountains in Virginia with some much newer and track-able V8 M3's and a 1975 Bavaria sedan (3.0 litre). Nothing broke or fell off. Now, I trust the car.
I bought my Z3 from a used car lot; it was a roll of the dice, but I found a list of things to look for in a used Z3:
The coolant tank tends to crack, along with the plastic radiator "top", which is actually the right end of the radiator (it's cross-flow).
The water pump often leaks, and the leak can l be observed with a small dental mirror, as the crack occurs on the back of it.
The rear shock mounts can crack
The differential mount can crack, although it's a very thick piece of metal - it would take some real abuse to crack it.
Look at the rubber parts under the car - CV joint covers, steering rack covers, etc.
Check for any leaks underneath.
It' a relatively short list. MUCH shorter than the list you would try to check on a Boxster, if you could actually see any of these things - I looked at 6 of them before I got scared off by the PPB system (Porsche Personal Bankruptcy)!
I'm not sure of your location, but here in the US, Z3's are not that rare. It would seem that you could find an example with far less than 170000 miles. Certainly, a well maintained BMW can go much further than 170000 miles, but miles are miles. Theoretically, the price you could pay should reflect the high mileage.
Good luck and enjoy the hunt!