Look at this forum for some insight:
https://forum.classicmotorworks.com/index.php#c1
That said, of the Indian-built enfields, you've got a few things happening. There are three generations of motor, termed differently depending on where you are/who you're talking to, but generally it's the pre-unit "iron-barrel," the pre-unit "AVL" which was a redesign on the same cases, and the unit-constructed "UCE". (Separate forums on the Classicmotorworks site for all types.)
I'd recommend you get an Iron Barrel, though many like the AVLs...the iron barrel is more traditional and parts availability is best, IMHO. As you note that a fuel-injected unit engine isn't what you're all about, the UCE bikes are all FI, though you can retro-fit a carb; believe Hitchcock's might sell a kit.
There are older 350s as well as the 500s. I'd recommend the 500 for US use, having owned both, but also wouldn't see the 500 as a bike ready for the freeway...so if it's back roads only, go with whatever you like or what's available. (Edit: my 535 with all the mods, aka the "Ace Fireball" package plus more, can barely make freeways tolerable, and even then it's certainly not enjoyable, just do-able...) With either, you're likely to want to get the crank balanced, which is the worst failure in Indian quality control. There are some treatments for rod and piston; "Ace Engineering" used to make some good kits and solutions including a quality floating bush, but I'm not sure where they are right now with availability.
The 4-speed transmissions aren't the best but certainly have authenticity...many convert them to run on oil rather than the oil-grease mix by fitting seals to the box. It's a right-foot shift by design, of course, and I'm told to beware the terrible linkage arrangement forced upon them for left-side shifting...that should go immediately if you get a bike with one, per most owners. (Never had a LH shift myself.)
The 5-spd is smoother, more reliable, less fiddly, and more efficient, and was designed to shift on the left, I think with some later iron barrels and definitely with the AVLs. It can be converted to right shift fairly easily, which is what I have on my bike. I got rid of the 4spd box when I did a ground-up rebuild on mine.
Indian-made points ignitions are pretty poor quality--people have adapted other points plates and you can find examples on the forum, though a basic Boyer is cheap and easy to use IMHO.
The TLS brake for the front is actually pretty good. Forks are basic and should be familiar--I put cartridge dampers in mine, though, and Hagons are readily available for the back. Air filters may need upgrades if you're making the bike any fiercer than stock.
Carb is an Indian-made Mikuni clone (24 for 350, 28 for 500) and may be better replaced with a real Mikuni or other quality carb if it gets troublesome-many of course use Amals. ("Mikarb" jets are not all interchangeable with Mikuni, alas.)
Oh, and the electric start is apparently a nightmare. Broken sprags for all. Many take it off entirely, which seems easy enough. Never used one myself.
So...the best model is probably whatever you can find, but if you find the "65" model, it has the 5-spd transmission but is pre-AVL era.
