If you don't already know that car would be the W116 chassis, which is the same as mine. The problem areas I have read about and experienced are related to the cruise control, power windows, A/C system, and of course, rust. For rust look carefully at the rocker panels and check unser the carpeting for evidence of water and rust. As far as the engine goes, someone else will have to chime in, mine is a diesel. The A/C system is vacuum operated and not as reliable as the later electronic ones. These "german yachts" are at home on the highway. Just FYI in 1975 they only made 6,167 450SELs, and sold for $21,709.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section