Keeping things STOCK

Ok so I want to truly keep everything STOCK. So far it has everything the Welcome Cassette Tape/CD the Original Window sticker in MINT condition. All the books everything! So in order to Protect the car I am thinking that I need to do some things so that I HAVE them and they get no more wear and tear. New Wheels and Tires might be a start and for that I think I want to graduate to a tire that FITS but is a Size larger. 17" might be a nice clean looking change. Here are some of my worries and concerns. 18" with a WIDE RIM in the back 9" requires a Fender ROLL so I don't want to do that because of Clearance and keeping the car Stock. 17" should fit and ride nice and provide a clean larger look. I may go without the WIDER tire in the back and then I could go with the 18". below is a Brief Idea of OFFSET I got this Info from TIRE RACK

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types (measured in millimeters).*
Zero Offset
The hub mounting surface is even with the centerline of the wheel.

Positive
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

Negative
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected.
STOCK BMW Z3 WHEEL OFFSET
Stock 17"s on a non-M Z3 have an offset of 41mm on all four wheels.

I also found this on BIMMER FORUMS and it is very helpful