From what I remember, the '98 model was just that, a '98 model.
Perhaps you mean the body style? What did you have in mind, the 4-door or the coupe?
The coupe was available from 1998 through 2000 in the 2.5 RS.
All 2.5 RS are N/A, from 2001 onwards the WRX was available which was when the turbo 2.0 became available.
You can turbo the 2.5RS using a kit, but keep in mind that it won't be covered by the insurance unless you declare it and if you don't declare it and have a wreck, they may refuse to pay out.
BTW, a google search for '98 Impreza specs can get you the full spec on the car.