There's rust and then there's RUST!!!!. If the truck has cancer (swiss cheese holes in the visible metal), you can bet that the places you can't see are shot. You're looking at a lot of work and money unless you can fabricate your own sheet metal pieces, weld them on and do your own bodywork.
OTOH, it's hard to find one of these that doesn't have SOME rust. The 3100's were bad for rusting out near the side vents. Mine was missing some of the floor and firewall. It's all fixable with time and money. HOWEVER, the more rusty it is, the less money it's worth, and if the frame is badly rusted (not surface rust), don't buy it.
It all depends on how mechanically inclined you are, how much money and time you have, and what resources you have access to. A farmer with a complete shop with metal fabrication tools, welders, etc. is going to spend a lot less money than a guy in the city who has to pay to get a lot of the work done.
My truck took 13 years and cost about $45,000, but I splurged on stuff. Lots of powder coating, complete engine rebuild with mods, cam, oversize pistons, etc., expensive paint job, professional interior, headers, exhaust, modified drivetrain, etc., etc.
