You should chance it. If it breaks its just another adventure for you, and a story you can tell. I would probably trust the bronco over some rental cars since they truly get the snot beat of them. I'll bet it would surprise you and run flawlessly for your trip. I'm sure theres others around that take longer trips in crappier cars.
You'd have plenty of room for luggage...nice comfortable seats...my vote is for the bronco. Make sure you've got the "junkyard" tool set with you, and extra of any fluids that it leaks, just in case.
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