It sounds like they got you over a barrel. I would ask them for a hard quote on what the body, paint, and interior work will entail, and cost. From that you can negotiate what work you need done now and what could wait until later. If you have a written quote as to what is going to be done, and what the price will be you can then hold their feet to the fire (so to speak). That would be much better than an open ended agreement to "keep it within my budget", but then "you should add this, or that", which blows out the budget.