My 840 has an offset of 6 mm on the front and 18 mm on the rear. This was done to "Fill" the wheel well with tire for appearance on my staggered wheels. Standard is 15 to 18 mm.
A 3 series has offsets around the 35 to 40 mm range by comparison.
Spacers are used to correct offset errors. To use 3 series wheels on an 8 you would have to use at least 20 mm spacers. That is 1 1/2" movement out on the wheels. This would be highly unstable to say the least.
Aspect ratios are explained in here: http://www.tirerack.com/tires/tirete....jsp?techid=46