Rob,
Here's a good website for general info on all Subaru models:
http://www.cars101.com/subaru_archives.html
To answer your question, yes the 98 RS 2.5 is very similar to the 96 2.5. They are both DOHCs. The biggest difference is 10 extra horsepower that was added in 1997.
Dwight
Rob,
96 was the first year for the 2.5 and no I don't have a wire harness (it along with the 2.5 engine now reside in my VW Vanagon!) I got the ecu as a spare in case I screwed the wiring up and burned up the ecu but it's been doing fine for almost 2 years now and I don't think I'm going to need the ecu.
Dwight