Yes there are, www.skylineowners.com, www.gtr.co.uk, the UK has plenty!
Skylines are pretty cheap over there too from what I read.
Are you talking 400hp at the flywheel or rear wheels? 400rwhp (300rwkw) is possible just not with the stock turbo and the engine will not last long on stock internals. You would want to strengthen the internals. Read around on here for more information. Ive never done it so i dont know.
Fuel consumption is pretty average I suppose, my fuel gauge is sitting on the line just before E and ive done 370km for this tank. Lots of WOT though.
Handles great. I dont know what you mean by "does the RWD still handle great"? Didnt anyone tell you? RWD has always been the best
Whether parts are expensive or not is up to your local scene. I find parts to be fairly expensive compared to say a Aussie made car but cheap compared to other imports. I buy a lot of stuff off the forums though and its a bit cheaper that way.
I service mine every 3 months or every 5000km whichever comes first. It really depends on how much and how you drive it. Also depends on what fluids you are using etc. I could probably do it less regularly but I like doing it so...