For me, it would have to depend on the condition of the car and condition of the two cars you would have to sell. Another question you have not really answered is whether you really want the car. I have sold or traded many cars as I got different things so for me, it is not a big deal. It just depends on how much you want the Mustang and how attached are you to your current cars. Hopefully the family will come down in price, especially since you are family because most people know the best way to loose money is sell a car that you paid somebody to rebuild. So to charge you the restoration cost is a bit out of line. Another question to ask is how long it has been sitting because you may need to do a bit of work if it has not been driven in a while.