I have a 1969 dated underhood AC kit for a 390 mustang, and as far as I can tell all the parts are identical to the factory stuff. I have never seen the underdash kit NOS in 40 years of buying parts ....
As for adding options, If your planning to sell at any of the big auctions, I don't think it will hurt at all (not alot makes sense at the big auctions anymore..) as long as the options are added correctly. My "R" code 70 was fairly loaded to begin with, and I correctly added the remainder of the options when I did the car. If I ever sell it and the buyer has an issue, I'd gladly remove the added options. ( Tilt, Convience Group, Intermittent wipers, Factory Tach, space saver) I already removed the front bumper guards as look pretty out of place ...) The car was a Dealer Demo for a local dealership so it was pretty loaded already..
Options like PS, Disc Brakes, and working AC make the car so much nicer to drive, I'd bet that for even a car that's restored to a fairly high level, for every buyer that was turned off by those options, you'll find 10 that will welcome them. A friend has a 70 SCJ thats a 4:30 gear manual steering, drum brake car and it's actually not very pleasant to drive at all. Try parking a manual steering 428 car with F-78 tires and a rimblow, and brake fade has a whole new definition with 10" drums.
JMHO, John