not just now but 5 to 7 years down the road

I really recommend you don't get a Z3.

Problems include embarrassment and a total lack of speed.

If you must go BMW get a Z4- they're much better.

Otherwise get a Honda S2000, a Mercedes SLK, a Mazda MX5 or, for more thrills, a Lotus Elise.

As for a more serious answer to your question, the Z3 has quite a good service record. Remember though that it's a sports car so has probably been driven harshly and will need the gearbox and clutch replacing before a normal car.

Note too that you can't expect reliability 7 years down the road when it's 13 years old.

Hope this helps.