We would probably plan for 2-3 months, and I would much rather see the country rather than spend the whole time driving, so a round trip wouldn't be something I'm too interested in. I'd rather spend time seeing each place than rush through it all.
Didn't think of the extra mileage that would pop up... I think a big car would be the way to go, like Dave said, easier to get around towns and also cheaper on petrol. Seems to be the way to go!