The South African Big Brake kit (front wheels only) is pretty close to
a simple bolt on...at least for Syncro Vanagons. I can't speak of the
issues with an older, and especially (former) aircooled Vanagon.

That helps for the fronts. Of course, yes you do have to convert to 15"
wheels to accomodate the larger discs and calipers.

For the rears, I understand that some places have a larger drum, but I
went with SmallCars rear disc brakes. I've extremely satisfied with the
result (big brakes in front, and discs in the rear). Largest load I've
carried so far since the upgrade was around 3000 pounds (split between
a trailer and the inside of my Westy). Braking was not a problem (yes I
did have to give myself a trifle more room, but less than with 1500
pounds of load with stock brakes).