nismos are nice but for the kind of money they go for even second hand there is a world of other exhausts out there that sound just as good if not better.
i'd be looking straight at the tannabe hyper medallion to start with, some of the ARK stuff is good as well. Invidia have some great systems. at the lower end the likes of cobra are very good VFM.
but what ever you do dont put crap speed on it. stay away from the K1 yes its loud but its unrefined and not in a good way. buddy club offer a far better option.
be warned on the nismo exhausts that there have been quiet a few occurances of the flixi joints needing replacing, as they have warn out, split and come apart. although it seems more common on lower cars where due to the nismo design the flexi can catch on things like speedbumps etc.
personally i think the nismo is over priced for what it is. but then thats just me, and people buy it for the badge.