R31's (at least pintara's)
You do have sway bars, they're the round bar that's held on to the front cross member with the rubber around a metal collar.
Heavier duty ones will be thicker than the about 10mm ones on R31's.
Upgrading wheels, don't know about clearances between stuff, but legally you can go 16" by i think 7 or 7.5" (what were the skyline silhouette rims stock?) but it's one inch higher and one inch wider.
Suspension won't need much if you compensate for the bigger wheels by putting lower profile tyres on - unless you plan on keeping on bush bashing, then put higher profile tyres on to keep the sump away from those pointy rocks (personal experience)
just common sense, really. If in doubt get yourself a good mechanic and ask him, that's what I do!