Hey this is tough. Everyone has their own personal "comfort threshold" for rust, to be sure. And depending upon what part of the country you're from (or live now), there's dramatically different ideas of what is a "rusty car." I grew up in Upstate New York and Michigan, so cosmetic rust/oxidation on vehicles was just part of the normal landscape.
That said, I have some iron-clad personal rules for the areas/amounts of rust on a vehicle that (for me) are OK, and which areas/amounts are "deal breakers."
1) Frame rails/suspension/axles/etc have a "coat of surface oxidation" that doesn't extend any deeper than the cosmetic outer surface. (If you took a grinder or aggressive sandpaper to it, you would reveal clean metal right away.)
2) Body: chrome/paint isn't perfectly shiny anymore. Possible tiny oxidation/rust bubble or two here/there under paint, but these need to be aggressively investigated to be sure they don't indicate actual internal rust.
3) Cosmetic (surface only) superficial rust on exhaust components and brake lines
1) Frame has actual ROT. Metal comes off in your hand in chunks. A grinder would actually go THROUGH the part if you attempted to clean it up.
2) Vehicle's Body is weeping rust out of weld seams (visible on top of paint!) If you see rust coming out of a body, run away. The cancer on the inside is likely incurable and widespread. You're looking at a nice coat of paint holding together a weak/rusty "van-shaped pinyata."
3) Brittle with rust brake lines / Frozen-by-rust e-brake cables
* Rockers/lower body on these vans (and also gutter area/roof rails) are areas in particular to look for the crusty brown cancer to have done their worst. Repairs in those areas are not easy / not fun.
* Introducing a vehicle that already has these "deal breaker" rust issues to more rain/mud/snow/road salt will just accelerate their rapid demise.
* Look at the hardware and nuts/bolts holding the shocks/suspension/exhaust hangers/brake lines/etc together under the vehicle. Do things look like you could actually disassemble/unbolt anything without them "snapping off" in your hands as you did so? Sure, a torch now and then is necessary to free old hardware, but give stuff a long hard look and ask yourself if you think the vehicle is actually practically serviceable....and not an adventure in torching/cutting/replacing all hardware for every job that should have been simple.
* If you're a determined welding/fabricating/sheet-metal/POR-15 wizard, then anything can be fixed....again look at what 86Scotty did with his van he nicknamed "Rusty"....but ask yourself honestly if you are "that guy," and is that how you want to spend your time/efforts?
Good luck/good strategizing/good vanning, good buddy!