I'm not an MB expert but from years of restoring old cars I'd say it depends mostly on the environment you drive the vehicle in, and what degree of "correctness" on the car you are willing to accept. In my experience, a urethane like POR15 will provide better long-term protection vs. an epoxy against rust in severe conditions, such as driving on salty, snowy roads. You can paint over POR 15 with modern auto paints.

I would not worry about compatibility with any waxoyl-based products, IMO it's long gone on any exposed surfaces you are about to recoat. (unless they have reformulated it, Waxoyl was just a mix of oil, turpentine and wax...)

But of course MB didn't use POR 15 when they first built the car, so using it is a non-factory modification if you look at it that way. If it were me and I lived in Wisconsin or some hellish place like that and drove in winter, I'd use POR 15. If I lived in southern California I'd refinish with factory paint.

I would heed the previously-stated advice to check the surrounding metal for damage also, and if you do decide to apply the POR 15 yourself be sure not to get any on you or anywhere you don't want it to go...

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1999 SL500