A common myth about fiberglass bodies and steel doors is that the doors will be too heavy. They say that about Scouts, Jeeps and just about all other off-road vehicles. It's just not true. I have put steel doors on various types of fiberglass bodies and have never had a problem - it just takes a little extra work.
First, lousy glass bodies are a waste of money. If they're flimsy and lightweight they'll just snap when you get them off-road anyway - huge waste of money. Invest in a body with thick, heavy floors and side walls. They should be reinforced with steel, checkerboard plywood (preferrably marine grade), or a combination of both.
Second, when hanging the doors, use a little ingenuity. If you can't bolt the hinges right through the side walls (like you can on a Jeep) you may want to fabricate 90 degree hinge brackets. These will place the weight of the doors on the vertical walls (very strong). You may want to join the top and bottom brackets with a custom fabricated bracket (made out of flat stock) for increased rigidity. This, of course, goes on the inside of the vehicle.
Don't rely on screws or bolts for mounting anything heavy on fiberglass. Use the natural strength of through-bolting through the vertical walls. You'll love the rust-free life of your new vehicle. Oh yeah, use stainless steel bolts.