I've seen the list of different pokemons between black 2 and white 2 (I picked up B2), but I mean, beyond those ones. Which pokemon can you NOT get in this game?

For example, I know you can't get a Charmander (outside of trading). Is there a list anywhere of all the 'mons I can't get? How impossible is it to actually catch over 600 pkmns in this game?

The only thing I could find even close to the answer you seek is this list from the first B/W games (bottom of the page)

http://www.psypokes.com/bw/oldpokemon.php

I can only assume most if not all of this list would be the same in B/W 2

I think you can get a bunch from breeding, but it's not something I've looked into. That may screw with the idea of a concrete list though.

Here you go: List of Unobtainable Pokémon

Also: Version Exclusive Pokémon

Serebii.net is a really good resource for all that sort of information.

Koffing in it without that stupid pikachu, sounds like a good collection in this after they ditched all the classics. Still seems like more than enough pokemans in the game that anyone should be okay on finding a good squad to work with.