JPL, I don't think 18" wheels and tires are going to fit, and if they do that car is going to be harsh on bumps. True it might handle better but I doubt that you gonna like them even IF they fit without scraping the hell outa the backside.
I suggest you stick with 15 inch stock wheels, the flat face (15 hole) factory wheels are great on that chassis.
Dieseldiehard
1971 220 (gas) 4-spd manual 104041
1979 300TD w/ ’85 turbo engine 288975
1983 300D 237300
1985 300TD 205437