age vrs. milage

I don't think this is a mileage vs. age consideration, the Z3 and Z4 are completely different vehicles! You probably would like one or the other, and that should be your primary concern. Drive them both and it should be quite apparent to you. Both cars, if not abused, should have plenty of mileage left to be enjoyed. Since you did not post the specifics of each vehicle, that's all the suggestion I have to give.

I agree they are 2 different cars, however, there is a large difference in milage. Can I expect the same life span (assuming systematic care). I really like the amenities and upgrades of the Z4 but the excess milage scares me. I plan on driving it for 3 yrs. and then giving it to my son, who will hopefully get another 3+yrs. out of the car.

Neither, as there are too many variables. The biggest is care/service. Why does the '99 have such low mileage & how was it stored all these years. Keep in mind lack of use can be worse than higher use because it may have received less care when it should have had more. Parts rot & fluids break down over time, so more care not less is needed. Most Z3 owners are fanatical about their cars, but it is a love/hate relationship. While cars today, especialy BMW's, are built to last the miles add up. I'd want to see full service records before consideing it & I'd want a full inspection from a local BMW dealer. If it has had superior service than I wouldn't let the mileage scare you away. Z4 owners are a little different, but many traded up from Z3's. THese are both very different cars and the choice needs to be based on which one you really love from a model perspective. Just make sure you have detailed records of service on the one you finally decide on.