You are correct that flat tow braking systems are essentially universal so yes, the Brake Buddy Stealth Supplemental Braking System # HM39530 should be able to work on your 1973 Mercedes-Benz 450SL since it has a brake pedal and a 12V battery to power this unit.
With that being said, I'm not a fan of the Stealth system at all. It can work but when I've spoken to our install team about this it's a bit finicky to install so they really don't like messing with it. I have some alternative... view full answer...
Gen-Y 10K Adjustable 2-Ball Mount with Stacked Receivers # 325-GH-315 is suitable for your application holding a winch mount and winch for pulling cars onto your trailer.
In this horizontal line pull configuration the winching force/car weight will appear to the ball mount to be just like trailer weight so the ball mount can be used with up to a 10K load, or up to the capacity of your winch mount, winch or its cable or strap. Naturally the hitch on your diesel pickup also needs to be... view full answer...