Hydrovac is the vacuum booster unit that makes manual brakes into power brakes. Your W200 should already be power assisted - by the vacuum booster on the firewall behind the master cyl.
Disc brake conversions on a W200 are about nonexistent. There may be a few, but I don't recall who anybody is that has done it.
Note - if the towing vehicle has better brakes and is stopping faster than the trailer, then the trailer will push the towing vehicle into a jack knife crash. It is 100% wrong to try and stop the trailer with braking effort by the tractor.
Plastic trucks suck...
Iron trucks rule...