Where can I get an accurate value for a car?
I would be wary. I would request that he find a battery and start it up for you, otherwise he could just be blowing smoke. I did a local car search in my area, and there were only 2 1971 Mustangs, one for $3000 that was... well, I didn't even see wheels on it. Then another for $18,500, but this one claimed to be completely restored.
I would suggest you look at the Nada or Kelley Blue Book--these are used car pricing guides that are the industry standard. I believe that there are "Classic car" Nada guides that take into account the level of restoration the car has/needs. If the floor is rusted out, and you find that the car doesn't run, then you can claim that the Mustang is a "project car" and get it priced appropriately. I really don't know how much it's worth, but you can perhaps, find some classic car clubs in your area, and ask one of the senior members what he thinks of the vehicle (My husband belongs to a classic Chevy truck club here in Utah, and those boys can spot a deal a mile away or tell you that the restoration needed is far more money than the car is worth).
Hope that helps, and good luck!