Assuming you're in the US - 1990's GM cars are dirt cheap to work on. Between RockAuto.com and your local pick and pull you'll be spending pennies on the dollar vs traditional dealer maintanance.
I hate so many things about how they are designed but for your criteria (diy, easy, cheap, documented) they are hard to beat.
Id say Toyota Tacoma. Super simple. Easy to work on. Large support community and highly available parts. Plus extremely reliable and insane resale value. So insane really that you're usually much better off buying new (assuming you know how to purchase near invoice).
It's not the most practical of vehicles, but the Mazda Miata, especially the 1st and 2nd generations (1990-2005), would be up there. Large enthusiast community thanks to spec miata racing, and parts are cheap since they've always been entry level vehicles.