in my opinion 18's don't belong on an e36 for a number of reasons
1. they are too big for the car, yeah they fit, but they look stupid
2. they are heavier, and they look stupid
3. the tire prices, and they look stupid
4. they are just too big for the car, 17s fit really well, 18 looks like you are desperate for attention, and they look stupid.
just my humble opinion, current market value: 0.25 grains of salt