Step 1 - Find out category you want to enter
Step 2 - Work out the class in that category that suits the car most
Step 3 - Build car to category rules
Step 4 - Buy tyres, petrol, parts
Step 5 - Enter events
Step 6 - Repeat Steps 4-6
You don't need a CAMS cage to screw around on track days (unless that has changed since I left Oz) so building the car for sprints or to some imaginary ideal spec is not a good idea. You may find that a car that runs such narrow wheels stock is not able to run 9" rears as R338OY suggested, categories have very strange rules that mean you often have to do weird things, so don't go out buying wheels or GT-R wings before you know you can use them.
Guys like Roy know a lot about these sort of rules so his advice is very useful.
Personally I would stick an RB25DET in it, stock brakes (which Sydneykid says are fine), big radiator, big oil cooler, some gauges, a seat and a GT-R front bar, BN Sports bonnet and a stock GT-R spoiler with some extension to raise it up a bit. Then the widest wheels I could fit under the arches with the narrowest recommended tyre size then lower it lots so the roll centres are wrong (just to annoy people).