KBB is a good starting point, I didn't input any options since I don't know what the car you're looking at has or doesn't have other than what you posted. You have to see what people/dealers are selling Mustangs for in your area. Value and worth are usually 2 seperate things when it comes to cars. Here in NC Mustangs that are in good/great shape go for more than KBB estimates. Around here I have seen people on craigslist asking less, but those are probably a big money pits.
Yes, but when you're talking same year cars, GT most always will be more than V6 even if it has more miles. (unless the GT happens to be a total P.O.S.)....But you won't know until you see it person, thoroughly check it out, and test drive it etc....