Plugs - Stuck with original brand and type as specified in the owner's manual. The engine will run best with those plugs. Stay away from all the fancy, overpriced, "split electrode, plutonium-tipped" plugs.
Oil - If you seach posts about oil on this board, you'll find way too many opinions on this. Again, check the requirements of the owner's manual and follow it, with the exception of stepping up the oil change schedule to every 3000 miles. I see nothing wrong with using a brand name oil, perhaps 10W30 weight, that meets SJ requirements (noted on the bottle). I believe in '85 the best oil available was one that meets SF requirements. Today, nearly all oils meet SJ requirements, which is a higher standard than SF.
Coolant - Change every two years, regardless of mileage. DON'T use the green Prestone-type fluid. Use Mercedes-Benz approved coolant.
Brake Fluid - Flush every two years with fluid that's required in the owner's manual, hopefully the DOT4 type.
For all other filters and repair parts click on the Fastlane link at the top of this page to order quality parts by mail at prices below dealer prices. It's worth it.
Post on this site to get the best advice for any other issues your son may have with his car.
95 E320 Cabriolet, 131K