I'm assuming you are looking at an '86 or '87 300SDL. If you happen to be looking at a 350 run (don't walk) away from that machine! Review all available records making sure it got regular oil changes (if they're not available check the compression), make sure the trap oxidizer has been removed, use Delvac 1 oil only and change it regularly and don't worry about the aluminum head. The SDL you're looking at isn't even broken in yet!

Also, check the trunk/rear window area and lower body cladding area for rust. If your mom is truly going to keep it for 20 years, you'll want to start with a clean slate there as body work can be expensive.

Good luck and happy motoring!