You should also ask each what they plan on doing. How many hours and the shop rate. Expectations should be clear on both sides. Is it OK to use rust converter on some areas? If so which. You would not be a happy camper if you were expecting new metal and they used rust converter. Not saying it doesn't work, just saying the cost is way different. Twer it me, I would look for one in better shape if I was going to farm the whole thing out. Maybe put some feelers out and do serious online looking. Once thing you can count on is there is more work where you can't see.
Dennis