If you really wanna go offroad, just strip down a 1980's suburban to the frame. Put in a big block V8, and then go to a junkyard and get a working powerglide transmission, transfer case, and huge differentials. And make the frame of the car stronger with steel tubing. All that is left after that is to make a wood mockup of what you want the body to look like, and make it one big fiberglass shell. Might wanna wear a scuba diving suit while driving if you plan on going through huge puddles! :P