I've already made a couple modifications! I was scared of making these modifications because other MB owners would more than likely call it blasphemous, but, according to the owner's manual of the 450 SEL, an MB owner can customize their vehicle to fit their needs (it's somewhere in the book. If I had it with me, I'd quote it in full instead of verbatim with page numbers).
Because I have to constantly drink liquids while I'm driving and got tired of my soda pop bottle rolling all over the place, I bought one of those cup holders that you put in between the seats and marine velcro strips so that way I could secure the cup holder without making holes and be able to remove it in case of a spill, etc. Of course, there wouldn't be a spill since I only allow travel mugs and soda bottles. I still walk into the restaurant when I eat...it seems to keep the car clean.
Second, I had to update a few electronics. I got one of those HD Radio/Satellite/iPod hookup/cd mp3 player/bluetooth (so you can hook up up your phone) radios. I have also added GPS (not in dash version) to my car, but the only place I could put it was by opening up the ash tray and let it sit there. I didn't want to destroy the dash and I can't reach the window to set it up.
As for improvements, I wish my car had a mirror on the passenger side, cup holders (front and back), and a better designed heater/ac unit that only a rocket scientist can figure out (2 pages in the owner's manual on how to work the a/c - 2 pages!). Better gas mileage would have been a plus, but it was the sound of the V-8 that won me over. One accessory I wish I had was a bug guard on the hood (bonnet) to prevent gravel from hitting the windshield. If the car had more room between the ground and the undercarriage, then I wouldn't be scraping the car on those large humps no matter how slow I go over them (<5 mph). It would have been nice if there were ash trays with lighters in the back seat area (still thinking 1970s technology and the fact they would have never guess the ever increasing set of electronics that we have now).
As for the other stuff, I'm amazed daily of the technology in the 1975 450 SEL: electric windows (working), electric sun roof (working), cruise control (not working), central locking system (not working), fog lights (working), automatic antenna (working), electric rear window defroster (working), child proof windows (don't care if it works or not as long as I can roll them down), and intermittent windshield wipers (better be working: I depend on them!)