Like what a lot of others have said, get the commuter car and build the 4-runner for whatever type of wheeling you enjoy doing. This is exactly what I've done and it works out very nice. I've got a Subaru that gets great mileage (~22+ in city and about 30 on long trips between 65-75MPH), and can still drive it all winter long and never worry about getting stuck. I've built my XJ for the type of trails i enjoy... It is still street legal though (sort of), because I have yet to break down and buy a tow rig... but with the killer deals on trucks right now I'm seriously debating it.
You can check KSL also, I see a lot of older mileage getters for under $1000 all day long... and as long as they were half taken care of, the mileage isn't that much of a deal. I have close to 250,000 on my subi and should easily go another 50K without even worrying about it.