The BMG rules are still downloadable on the website, but they've not been updated to match the printed rule book, so I would suggest avoiding it. If at all else, buy the rule book and proxy the models as you like. The game is sound and for you and your friends, who says you have to use KM's figures?
That said, the models are expensive on a price-per-figure basis, but psyberwyche is correct. You can play games with anything from 2 models or more, depending on the game size and figures you choose to play with. I have hundreds of Heroclix and like using them for miniature gaming after they've been repainted, but for the most part they certainly don't compare to the quality of the Knight Models stuff.
I think two boosters of Heroclix will cost you more than a single KM figure. Sure you might get 10 figures, but the quality isn't there, and you might not even get the model you want.
It's up to you.