Hi mate, I seem to remember someone saying that Apexi downpipes are the best (if you can still get them!). Although personally, I'm going to go for the all-in-one trust elbows and downpipes:-
As for exhausts, well, personal taste has a lot to do with it in terms of noise level. I hear a lot of good things about Kakimoto but theres also Trust, HKS, Nismo, Apexi etc etc....
Might be a good idea to see if there are any local skyline owners who have different exhausts and arrange to have a listen. Or, go to a show where there will be loads of skylines, perhaps with a variety of exhaust solutions.
Hope this helps
TT