Go for it! I think it sounds like a great trip!
My "Great West Tour" in 1997 out of college utilized a very rusty $1500 1980 Ford Bronco with 170,000 miles on it. I bought from a sheep farmer in Decatur, Illinois. (I think he hauled sheep IN the truck.)
My trip lasted 3 months and 18,000 miles.
ONE breakdown. Fixed during an ice storm with a Swiss Army knife in the Dakota Badlands.
Take the time to do the truck repairs and maintenance now. It'll be cheaper, faster, and easier than doing it on the side of the road. If I can go 18,000 miles in a FORD and only have a fan shroud fall off, a TOYOTA should be able to cover 5,000 trouble free miles no sweat.
Get that fuel tank fixed up first. It is the thing preventing you from going and probably the most expensive problem. Put in a leak-free, rust free tank. Save you from a lot of fuel system problems on the road.
It sounds like there is a lot of metal missing on the body. You not going to hurt the value of the truck by slapping some functional, hand-made repairs in place of the missing stock steel. Use angle iron and plate to reinforce stuff where you need structure (fenders?). Then just get a sheet of steel you can cut with tin snips to cut and fit patches over the non structural rust holes.
Don't underestimate the strength and speed of repair you can affect with fiberglass mat and gel. Maybe replace the missing structural parts with steel and your welder, then cover up the sheet metal holes with fiberglass mat.
Indoor/outdoor carpeting is a cheap way to cover up your patchwork and the smaller holes you didn’t get around to patching. Makes your interior look nice and clean again, and helps a TON with road / transmission noise. New wal-mart seat covers to match the new home depot carpet, a little spray paint work here and there and man, you’ll look downright GOOD while on your adventure!
I didn't use my radio much on my trip, I used my harmonica. Learn a new instrument while you enjoy the scenery.
Yeah. Earplugs and a kidney belt would help too.