Hmmmmmmmmmm...... mis-matched tires, aftermarket A/C,
horse shoes attached to the taillight panel???? Imagonna say $250, but just 'cause I'm a nice guy; I'd go as high as $500, providing you deliver it.
Now that THAT silliness is out of the way; the value of the car
could be very high; although I'm not sure you'd see a lot over $10k in this economy. The prime detractor in early Mustangs is
rust. Now, if this is a one-owner always-garaged car, things look pretty good; because that's the biggest rust-deterrent in any climate. However, any
really serious (like "$10k or more" serious) prospective buyer is going to check in all the usual locations: The cowl vents, the very front of the floor pans (that's where the water goes and sits if the cowl is leaking), bottom rear corners of the doors. In a vert, probably the trunk and rear seatback areas (if water has leaked into the "well" where the top goes when you're sitting in the sun). Battery tray (if a battery "burps", the acid eats metal); lower radiator core support; frame stubs front and rear. Original 4 spd tranmission? That's worth some money right there. Be prepared to watch a guy crawling under the car and in the trunk, poking at the floors with a screwdriver; maybe running a magnet over the body panels, looking for bondo. It's no mistrust over
this particular seller; just a healthy dose of "reality" from a buyer whose already heard the sad-but-true story of a guy who paid almost $25 thou for a '65 (or was it '66?) "PRISTINE" 'vert from a "reputable restorer/dealer" and found out the rusty shell was held together (barely) with fiberglass patches and old STOP signs; and was beyond repair.
If this car is as good as it could possibly be, and the buyer has money from somewhere; the seller could be making some serious bank and know that the car was going to lead a pampered life. It's all in the non-subjective condition of the car.