I had some stock BMW 17 inch rims on my 2001 Z3. I sold them and packaged them up and they weighed around 40 pounds with the packaging stuff included. That is a lot. These were nice rims also.
Also, I was thinking of putting 19 inch rims on my Z3. They will fit and not rub but only if you get a 30 series tire or smaller on a 7.5 inch rim or smaller. These types of tires are very expensive thou. I got 35 series for my 18 inch rims and they are fine.
If you are thinking about getting new rims for your Z3 just keep in mind that you may need to get different size rims offsets. Like a 18 X 7.5 inch for the front and 18 X 8.5 inch for the back. A lot of places don't like to sell tires unless they are the same size.
If I were you I wouldn't get any tires widder than 8 inches. unless they are on the back or on a M series. Widder than 8 will throw mud and crap all over your doors. Let me know if you want pictures of my Z with 18 inch rims. A lot of people say it looks great.
2001 BMW Z3
3.0i straight 6 engine
M badge series
Dinan cold air intake, Dinan software, Dinan High Flow Throttle Body, Dinan stage 2 suspension upgrade, Dinan Strut Tower Brace, 18 inch wheels, and don't forget the fancy pedals.