You might not have to buy the expansions, They might be mostly for the story side of it, and any new units can be added through a patch. SC2 will have a massive singleplayer, this Terran one will be as big as the whole SC1 story was, there is something around 30 missions which is why they added expansions.
I dont think its greedy of them when the SP experience is so huge, I used to say the same thing you are saying now, until I learned how big SP actually was.
The story for Starcraft is pretty good and I would be willing to buy all 3 for the story. $180? Probably not. Wiki is there for a reason. But playing through the story itself would be more fun. --- Check my youtube page out for the latest TF2 and L4D2 videos! youtube.com/user/7gamex