More info than you probably want to read on Transit wheels/tires on the Transit Forum. Lots of off-topic and arguments there though, beware.
From what I know there is virtually no difference between the Transit 150-350, really just spring rate I think. Motors, bodies and trannys all the same.
My Transit is 2wd so I'm hesitant to answer your question about what wheels/tires will fit. I'm thinking you can go up to 265's pretty easily though. I have 245s on my 2wd with no lift.
Currently vanless. Weird.