Rusty bolts are another problem. For those who are unfazed by penetrants, heat is one of the last resorts (the very last being drilling and re-tapping). Excessive heat will indeed warp thin metal, but if you're careful and apply the heat in a controlled manner you may well get away with it. The door hinge boltholes are normally backed by captured nuts that are equally as stout as the bolts so they'll help soak up some of that heat applied to the bolt. Just don't go turning them cherry red or you may get a bit of heat-related distortion in nearby sheetmetal.
Well, I hope this was of some help-but I'd suggest you take some time to sit down and figure out a plan of attack before your start ripping into it and spending gobs of cash, only to regret it later if you change course.