1. I haven't tried sleeping in the back of mine but I think I'd be able to if i put my head at the tailgate end and hung my feet over the end where the seats are, one seat would have to be forward and tilted forward also to be able to do so, but in answer to your question yeah i guess you could.
2. I'm talking in terms of the KE70 van i have here, some stuff is different, some is same as sedans. Say for instance you want little stuff like a headlight suround, corner light and a bumper bar, you will have an absolute pain in the arse of a time finding them, seatbelts too and doors as well (trademe NZ is a good place to find these parts easier than Australia). But if you were restoring one and weren't concerned about originality, you could put a full KE70 sedan set of panels on the front.
3. If you can pick one up without rust for anything under 2K you're getting a bargain, like anything if you're patient in your search you will find the right car at the right price.