Wendy, I believe norcalsurfgal is talking about a slot car set rather than the models Revell marketed. Some slot cars are worth far more than a normal model car kit. The 300SL (Gullwing?) I think would fall into that category. As a regular full size auto, that is one rare car (Gullwing) and worth many $$$. As an unassembled slot car, I would think that would bring a premium also. Slot car sets and cars are valued as to condition. Uncrashed cars are worth more than modified, user run cars. Useable track is desirable. Unassembled cars are collected with some scarce models bringing good prices. I would Google the specific brand of car, model, and condition to see what the current worth is.
If you are keeping this for your son, make sure he would appreciate it rather than just run cars around the track to crash them. The way to devalue what you have in a hurry is to assemble unassembled cars and race them. Already assembled cars can be carefully run around the track a few times. If you have cars that have been altered or crashed but are still operable, those are the ones to let your son race around the track.