Actually they made an 8 speed RSX as well as a 7 speed. I think you can get new speed Sora STi's, but then you've spent more than the $250 you were planning
The other thing is the 7 speed crank used non standard chainrings. I think they were 26,36, and 46 versus 30,42(or 39), and 52 on a standard road triple. I used to have a GT with the group, so I'm going off memory. The rear was a cassette with a spacer, which means it'll take 7,8, or 9 speed cassettes.