[MrPbody]

Pretty broad range of questions there guy.

Currently early '60s cars are gaining momentum, as the muscle car sources get played out. Finding the Plymouth in good condition AND for sale, could be a challenge. The Chevys are more plentiful, but also more popular. The upside of that is the availability of parts and information.
Unless it's all there, I would avoid 409 or 348 cars. Certain pieces are made of "unobtanium". The expense of rebuilding the 409 and 348 is quite high, as well, compared to the 283/327 stuff.
One of the more popular and rare models is the '62 "glass house" or "bubble top". That's a '62 2-door with the '61 roof line (big back window, thin "C" pillars). '63 and '64 Impalas are still pretty available. The big-finned Chevy is the '59. Rare.
The Buicks are gorgeous cars, but drivetrain parts and services are hard to come by. The Olds as well. They changed their engine families later in the '60s. The Pontiac is another with lots of chrome , grills and fins, through '60. Most later engine parts fit the early V8s. You'll find the value of these three GM division cars rising fast. Some of the special ones are already cost-prohibitive. A '63 409 dual quad car can go for as much as $10K for a rusty one, "ready" for restoration. Even more if it has a "history" (I have a '62 Catalina that was an NHRA record holder in the mid '60s. I'll be hangin' on to that one...).
May or may not be much help here...