First off, good choice! Have you decided between a coupe or a fastback? Obviously, coupes are cheaper, so you can throw more cash at the other stuff, but some just gotta have that fastback. (Cant argue either!) IMO if you going with a coupe, get a 65-68, fastback 65-70. The 67 and up mustangs have the advantage of more engine room (could come with a BB from the factory) a dual master cylinder, and the ability to run a dual MC and power brakes. I guess I would recommend the 67 if your going with a coupe, since you can run a power dual MC and it doesnt have the side marker lights (smoother). A 69/70 if your going with a FB. A downside to the 69/70 is that parts seem to be more expensive for them.
I actually cant recall others using Cobra discs on their classic mustangs, so I googled it, and sure enough there is a company that offers a conversion kit.
Vintage Venom is their name, and their prices seem quite cheap. I've never used them, or even heard of them for the matter, so you probably want to use caution if you go that route. Anything is possible, I saw a mustang at a car show awhile ago with corvette discs, and yes, I did take a picture, haha.
Did you want to do a bolt in suspension or something more? I just recently bought a Mustang II front suspension for my 65, which I will be installing this winter. Once Im done with that I can let you know what I think about it, though the earliest it will be on the road will be next summer. Other than that I've only used some grab-a-trac springs, so I really cant comment much on good suspension systems.
I dont think you really have to worry about if you get a I6 or v8 car, unless you dont want to change the spindles and whatnot. If you can get a v8 car, but I wouldnt sweat it too much if your ripping it all out anyways. The v8 stuff does sell much better though
