I say:
Buy the entire Baldur's Gate series in a bundle.
Play BG2 first. It is the best game in the series, and the best one to break you in.
Then play BG2:ToB with your BG2 character.
Then, once you're all invested in the story and rabidly foaming at the mouth for more BG content, THEN play the entire series starting with BG1 all the way through ToB.
I've always found BG1 more compelling in context as a prequel to BG2, rather than a standalone game.
As far as OTHER Infinity engine games, Icewind Dale has great action-oriented content. Planescape Torment has great story-driven content. Both of those games are worth playing for those respective strengths. But, Baldur's Gate 2 is the total package, a verifiable masterpiece.
edit: KOTOR is good too for what it is, but its not the timeless work of art BG2 is. Ultimately, you should get around to playing all of them!