First of all the value of this car is going to be based on condition,location,and what market the seller is presenting it to.That is to say the average buyer just looking for a car is not going to pay much for a car that old,on the other hand a buyer interested in restoring or owning classic cars sets an entirely different value ,as would a seller presenting it to that type of market.Kelly Blue Book would be of no value, it only gives the value of cars dating back to 1987.It is also important to note such a vehicle would sell to that type of market for much more in say California
than it would in say Oklahoma,because of pricing and cost of living and overall value of any item in a respective place, a $50,000 house in Oklahoma for example would sell for $150,000 on the west coast,all things being equal.The value should be determined by what the seller reasonably wants considering condition,and what the buyer reasonably can expect to restore the car,and final restored value.
As far as parts for the car, irregardless of the motor (the most common in that year (289ci) the parts will be readily accessible and there are even tons of aftermarket parts designed specifically for the mustang '65 & 66 models.
The most important issue especially if restoring is rust.If heavily rusted you are looking at lots of metal fabrication and overall stability of the car suspension.Of course the conditon of the motor & drivetrain(tranny & rear end) are important,but easily replaced or repaired with lots of parts available and crate engines ,aftermarket suspension and all that good stuff available.
In general you just want to access how much it is going to cost you to bring the car back to the level you want. A fully resotred mustang especially if muscled up would bring $50,000 at a classic car/muscle car auction.If you just buying it to have a car to drive that is old school then you would want to figure what it takes to get the car into safe reliabel running condition, and add either amount to the cost, and check your budget.If restoring in order to bring top dollar you have to bring the car up to top condition which would require at least $10-20,000 doing most of the work yourself and farming out the body & paint work to a good shop.If just for a car to drive,I would'nt pay much because your not going to want to put much into it.It is definitely a niche market car, primarily valuable to the restorer or customizer willing to put in the time and money to bring it back to original or better condition.