errr, you kinda get everything when sc2 is out, no additional fee, no DLC. seriously not sure what DLC/patch you're referring to.

The roadmap for Diablo 3 appears to be similar to Diablo 2 content wise. Four Acts in vanilla game, extra Act per expansion, fight Diablo in Act IV. No silly nonsense like cutting out Zerg and Protoss campaign to pad with filler for expansion SP if that's what you meant.

Its just you cant play as Zerg in Wings of Liberty, Protoss is only playable in a couple of missions. Terran is like the main star of the first game. Then, there is a second expansion coming out soon that you are playing Zerg, probably for the whole game. You say its like splitting the first Starcraft game to 3 separate games. Its a lengthy campaign, i would say, even for Wings of Liberty.

You still can play all 3 races in the multiplayer.

Diablo 3 should have whatever we have seen so far. I think Paladin or anything else will appear in the expansion instead but the classes announced will be in Diablo 3, unless Blizzard plans to piss off everyone.