Hi Steve,
The Z3 is capable of high mileage if kept maintained. The 6 cylinder engines are pretty much bullet proof if maintained correctly. The issues to look for at that mileage are rubber suspension components and anything rubber like radiator and induction hoses on the engine.
They are a perfect car for the average home mechanic, and pretty easy to work on. If you have access to a code reader it's worth taking along to a viewing to see if anything is picked up. It's mostly sensors that fail, and they are usually easy to switch over.
Any specific questions just ask.
Gary