As I understand, "GT" was just a trim level and some even had slant six engines. All GT's had bucket seats and floor shifter.
The car is special since, as mentioned, it had the rear cutout for the exhaust that only the 273 Hi-Po engines had (AllPar article, I recall), and special finned valve covers. Unfortunately, it appears the engine was changed. Check the casting characters on the driver's side of the block. In 1965, they did not list engine size, so if you see "318", "340", or "360" you know it is wrong. There should be a "65" in the characters as I recall. You will also find an engine stamp w/ "273" on the machined flat pad at the front of the driver's side head (if heads weren't swapped). The reason I suspect changes is that it has a later exhaust manifold (Magnum engine?) and the intake is probably for the later heads w/ 45 deg bolt holes. The 1965 273 had intake bolts ~16 deg off vertical. Some people (me) have hogged out the holes on a later intake to make it fit, which would be obvious.
Re price, I paid $1000 for my 65 Dart GT w/ regular 273 engine. It was rougher, but no big rust-thrus other than the passenger floor. It all depends on what you can live with. When I find the typical "black spray bomb over rust" in the engine bay, I feel compelled to strip and sand to clean metal, so the prior owner's "improvements" have negative value. I can't tell from the photos exactly what you would get. I separately bought similar Cragar wheels for $400, so up the value. Ditto for carpet $120, headliner $100, seats (up to $700), etc. That luggage rack is cool. I wonder if an original option or taken from a Mustang.