It's hard to estimate the value of a car without seing it but from what you say, I would not offer much more than $2.000 for a basic 68 coupe Mustang. Fastbacks are much more popular and the double of this amount would be correct. The replacement sheet metal is not very expensive but if you get it done by a body shop you will have many hours billed for the job and once the quarters and floors are removed you may discover much more rust damage.
You may also want to ask the door tag numbers to make sure it is a number matching car. Its value will depend of that once your car is restored. Once you have your door tag numbers you can decode them with one of the websites listed at
Google.
You can also check at Ebay.com to see how much Mustangs in this condition are sold. They have hundreds of them listed.
Good luck and feel free to post a message if you decide to purchase it.