DOT 5:For what it's worth
I put Dot 5 brake fluid in a 1977 Toyota Celica and drove it at least 80K miles over 7 years without doing anything else to the brakes except changing the front shoes once.
My wife had a 1980 Toyoya Corona and I replaced the calipers, the cylinders and changed the brake fluid to DOT 5. It went another 90K with nothing done to the brakes and would have gone further had it not been stolen.
I don't know whether ABS brakes would or would not be a problem, and therefore have not put DOT 5 Mercedes.
If it means that no water gets into the fluid, then the extra cost of the DOT 5 is offset by the elimination of changing the brake fluid so often.
If DOT 5 gets on a brake shoe, my experience is that that shoe will not lock up, as it will with DOT 3. Just rub the rotor and pad with MEK (lacquer thinner).
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"