it would be fairly simple to make a bobtail pick up out of a raingie but you say its going to be used offroad most of the time.......why do you need a heater? i had this revilation when trying to make my snorkle fit around the heater in the disco, dont need that, dont need the rear window i had put in, dont need the door windows as my elbow was always out the side(true landy fashion) and all of that weight does add up. if your that worried about getting cold, shunt nose it aswell and throw the rad in the back i hear that can get rather warm???????
you have to weigh up what your going to do with it. if its an offroad pay and play toy i would just remove all the fancy crap(maby get some dosh for it?) like a heater and carpets. bobtail it maby a little lift and some nobbily tyres and you will have alot of fun.
if you want to use it for pay and play and maby some events shunt nose it, put a full suspension kit on there, winches, and some bigger tyres and again you will have fun
if your just going to compeet either trayback it or just throw the body away and make a buggy then youll probibly need to spend alot on winches, tyres, suspension and your axles.
that is my view anyway. nothing is set in stone with this stuff ;)