Take a sanding disc to the rust and then do a POR-15 treatment in the trunk, underbody, floors. Toss a good two layers of paint over that then some underbody coating and the 240 will not rust away. I am surprised at how rust free it actually is.
Adjust your valves in both cars. The valve cover gasket it a $6 part and the car will run better after adjusting. If you really feel a burning need to plow cash into a car, do the 300D. Take some spare coin and bring the 240D up to better snuff by detailing it, getting fresh bushings and other rubber bits taken care of.
While the OM617 is being cared for, just drive the shit out of the 240. The more you drive it, the better it will perform. Take it to redline, up mountains, on the interstate, visit Maine or head to DC for some long, coke burning, drives to get the snot out of the engine.
Stop stressing about the cars and just drive them until you find a real deal buster that needs big attention. The 240 should be a daily driver to keep it in good shape. Do not granny drive it. Explore the envelope of speed and handling.
Detailing the car will also really spiff it up and show you the top quality of the paint. Wash, clay, polish and wax will make her shine in a manner sure to engorge your manhood.
Originally Posted by Mercedes Lover
Took a wet rag and rubbed it against the rust underneath the 240D. Rust came right off, just minute surface rust. Going to take an angle grinder and work the trunk and unibody, should be A okay.