As an owner of a 300E, I can attest to its indestructible nature. Since the car is pretty old, check motor mounts and driveshaft play. Ask the owner to show you some history of the car. All cars need some wear/tear items but there is no car that wont need some preventive maintenance. I think the best test of a 300E is a simple ignition at the morning when it is cold. If it starts up strong then you have a keeper. Open the hood and look at the engine. Idle should be strong and If it shakes and dies, it's liveable but it can be fixed.
This car is a mechanics favorite car. My mechanic owns one and he says it is the best and easiest car to repair. Summer is coming, so have the AC Tested and have someone perform a roartest on the engine to make sure for no overheating. If it overheats, repair is no big deal; either thermostat or water pump.