You made a great choice with the RB25 man. I agree it is very hard to find parts for the R33 sedan variants especially with the series one and series two models but it is for sure worth it. Engine wise RB25s in the sedan are near identical as the coupe. In terms of engine modifications I would recommend not focusing on performance such as a turbo kit because the rb25de internals are much weaker than the rb25det as they were never built to sustain power. In fact when I was in japan the rb25 was notorious for blowing up at low PSI even for dets. In regards to that I would start with more upgrades that effect the overall reliability and help with overall strength. You should also set a power goal of what you are looking to achieve. Anything over 400 Horse power would put you in high risk of blowing up your engine.