I love my 65 coupe but some would have only a fastback or convert. The early coupes can be had for less $$$ than a coup or open car, and a six cylinder is even less...most of the time. In the last decade or so the six has gained popularity and many have been modded for more power, but understand, the six can't be made to run with a V8 for the same money. I have owned both and now have a much modified 6-cylinder coupe and it would be hard to get me to swap it for a fastback. The 32 is to show I have been doing this car building thing a very long time...like 50+ years. The magazine story was in 1976.
I forgot to mention, the 6 can be made to outrun the V8 in the left-right-left road racing world, but requires lots of money. Mine has a rear sway-bar, poly bushings in most places where rubber was, front disc brakes, stiffer coils up front and leafs in back, 15" wheels/radials, and the motor is bored to the max with a 2-barrel, hotter ignition, head work and free-flowing exhaust. The 6 is much better balanced for road racing but the little Small Block Ford V8 is strong in the straights. When I am cruising the interstate running with the Big Boys in the NASCAR lane...they do not know it is a SIX...until they stop for gas and I keep on running.
that is what happens in soviet russia, the car picks you!
OP the range of cars you picked are my favorites. i own a 66 c code coupe. best advice i can give you is start by finding a mustang in decent condition that runs good enough to drive daily. if you prefer convertibles, then buy one you like, if you like fastbacks or coupes then the same thing. if it doesnt matter then buy the best car you can afford, that you like.
remember to check it over carefully for the typical mustang rust issues, especially cowl rust issues. once you have the car, start by driving it around and get used to the car before you start making a plan for the car. this assumes it is in proper condition for driving, Ie the brakes work like they should, it has god tires on it, the suspension is in good working order, etc.
once you have driven it for about a week or so, you can start to plan what changes you want to make. things like adding chassis stiffening, rebuild/upgrade the suspension, brakes, etc. do these upgrades on weekends, s that you can have the car to drive during the week t keep your enthusiasm up for the car.