What is the exact offset for an 88 325ix? Does it/ could it change for bigger aftermarket rims (such as 16" or 17")? Also, for winter use, can you safely use a set of 14" rims from a regular 88 325i?

The offset will change when you go to a wider wheel than the OE. Offset is just like an aspect ratio on a tire, as it goes wider the offset gets smaller. The OE offset is 38mm . We do have 14" steel wheels for the car for snow tires. You can see all the wheels we have for the car on our web site by searching wheels by car. You will also be able to see the wheels on the car. If I can help with the snows or your wheels give me a call.
Thank you Jim 800-428-8355-364