AC systems are not cheap, cheapest route may be the all in one hang under dash unit as mentioned above with the evaporator, controls, outlets, etc. built into one unit. That unit can be pretty much universal as the condenser (mounts in front of rad.) and receiver drier can be. The motor specific part is going to be the compressor mount/ compressor and pulleys as they must line up. Running the refrigerant lines and electrical can be fairly universal also.
I would still use the Sanden style compressor (need mount mentioned above) for it's compact size, efficiency and universal replacement and uses R134 freon. The original box style one is heavy, inefficient and harder to find as well as not necessarily set up for R134 as they were R12 units, the rubber, etc. is not compatable with both types of freon and R12 is rediculous $ and hard to find.
As far as vents you can still find them on fleabay, etc. or use universal ones made to fit mustangs or just use the ones built onto the hang under unit.
I plan to install AC in my '67 but will use most of the CAA kit, I've already colllected parts like the drivers side controls, center and passenger vents as I like the stock look inside and the required crank pulley all original parts. The engine parts will be the Sanden compressor/parts from CAA.
1967 or 1968 Mustang-Cougar Air Conditioning System | 67 or 68 Ford Mustang Mercury Cougar AC
Jon