If your work will ship it back free, you are not losing anything by bringing it back. What would you do with it if you don't bring it back - do you have a convenient way of selling it, and how would you ensure that you were not ripped off?
If you bring it back, you can sell it as is, do the repairs and sell it, do the repairs and keep it or not do the repairs and keep it. To make those choices you would first need to know the cost of the repairs, then the value with and without the repairs, and then know how much longer it would last without doing the repairs at all (with rust, if you are going to do the repairs, do them quickly or not at all).
Dots of rust such as you are describing, on the hood, roof and passenger door frame, sound to me as though it may have been parked outside somewhere it was subject to a particularly paint-eating tree sap or bird poop and then not cleaned on a regular schedule. If that is the case, the undercarriage may be OK.