I have a '93 300CE (essentially the same chasis as your 88 300E) and I have fitted 17" x 8" wheels with 225x45 tires. The offset on the rims is 42 cm. I don't have any rubbing problems, however, I doubt that you can put on a wider tire than a 225 without running into difficulty. Once you get up to this width, you have to be very careful about the offset of the wheel. Too little offset and the tires will stick out of the wheel well and lead to rubbing against the body panels. Too much offset and the wheels will rub against the suspension compenent inside the wheel well. Using an 18" wheel brings other complications. Your tire needs to be very low profile (hence very costly) in order to maintain something close to the stock diameter. Very low profile tires give a harsh ride and don't provide much protection if you hit a pothole.

Eric