You have to modify the wrx turbo because it will not go below 7-9 psi of boost (Wastegate pressure). The absolute cheapest route would be to use turbo legacy parts. This will spool even faster than the WRX turbo. The usual parts list for peicing together parts from a turbo subaru is like this
-Intake and intercooler (i suggest getting a new intercooler as turbo legacys came with water-to-air intercoolers)
-Oil and Water lines for turbo
The 98RS has tiny injectors compared the the 99+ so you are going to need a RRFPR. At 5 psi of boost i'd doubt you'd need timing control, but it couldn't hurt to get something like a J&S Safeguard.