Definitely worth keeping on top of any bodywork and you probably won't have to carry out much maintenance to the rest of the car as they are pretty bulletproof.
Mine was unfortunately written off by a Transit van on the wrong side of the road :-( However, I bought it back as a Cat C and replaced the front bumper and the bonnet and all okay now except for some fresh paint need on the front end (wrong shade of silver paint).
Otherwise, apart from servicing and a front caliper which seized, I have never put a spanner to the car, it never needed anything to be fair. Great reliability!
Any advice you need just ask as I'm sure there are plenty of experience Z3 experts on here who can help