Drop in the turbo RB? Can't go wrong with that.

some more info from the owner:

The engine is the RB25 Neo (with VCT) with a 3 inch exhaust with silencer and a new pod filter air intake. The final plan for the car will be for street use as a daily driver with the occasional track use with the end result of the car utilising using a small supercharger.

Supercharger on an RB? That's something you don't see everyday, but so far there's no really good supercharger builds out there at the moment. Can't go wrong with the GTR's twin turbo system for daily driving and big power.

there's heaps of mods available for the nissan's.
a turbo motor would be the obvious (but expensive) choice. Other than that, you'd be looking at something like a tuneable piggy back ECU, upgrading the extractors, high flow cat, a proper cold air intake system going to the pod filter.
it all depends on how much you're willing to spend though...

for the track, suspension upgrades, brakes and tyres will make a world of difference though, sometimes even more so than more power.

If your going a blower, go a tuned lengthe 6 into one collector extractor (Custom made will be needed so a bit bricey but great tourque on the street) get some suitable cams made up, better valve springs & valves, no necisarily biger but stronger. Personally I like the blower idea as its different to the usual drop turbo engine in idea

of course its different, but with different comes the price.
for the same power I'd estimate paying double the price for a charger onto the RB.
you then come into factors like the turbo is proven to respond well on the RB when the charger isn't.
Comes down to how much they're willing to spend in the end and to make sure they go to the right shop for it.

As Xlr8_87 has said. Concentrate on your handling first as this is where you will feel most of the difference in an otherwise standard car. Do you want to turbo the car in the future?
If so you will be looking at upgrading things like intake, plenum, engine management, fuel system (injectors & fuel pump) and of course purchasing all the necessary goodies for the turbo and cooling.
Alot of what you would look at upgrading for the addition of a turbo you might also look at upgrading if you were to go mental on an NA RB engine as well.

I guess the one advantage you have on your side is that there is a wealth of knowledge out there for the Skyline and RB motors.

If you're going for a street-legal racer, then really like Steve said, go for the handling first. Decent suspension setup and some stickier tyres can turn any car into a nimble beast. If you have the cash for it, either get a twin-turbo setup or transplant the RB26DETT into your 'Line for some extra power. I don't really know the details of what else you'd need to replace, but certainly it's something that can be easily Google'd. Skylines as you may know are really popular tuner cars, there are plenty of ways you can go about giving it some extra juice, but as for supercharging...I don't know if that would work out very well. Turbocharging is really the better way to go for RBs.