For 32s;
1; The RB20DET is the only engine option for a turbo 32 (non gtr), but this is offered in both two and four door guise, as well as two weel and four wheel drive. (GTSt and GTS4) 160kw and 263 Nm.
2. Some rust can be a problem with earlier 32s, in the boot and under the rear guards, also there is an extensive guide on here to purchasing a skyline (try a search)
3. Exhaust,
high flowed or aftermarket turbo (ceramic tubrine is quite an achile's heel)
FMIC (as SMIC can be restricting)
ECU, (Power FCs are popular, although the AP engineering ones for 32s, are more expensive, and harder to come by)
Boost controller
Fuel system; injectors, pump, fuel pressure rise regulator
Clutch, brake pads (possibly discs if circuit work is intended)
After that cams and internals are generally next on the list.
4. R32s, depending on which type you are after (GTSt, Type M, GTS4...) Can go for as little as $10K, but $12-15 would be reasonable, $15+ for something special or immaculate, although at this price, 33s in good nick are also options.
5. I have a set of Tein HAs. Almost every other car i have used since getting my 32 feels like a boat.
6. Love the shape, and they are great fun! Whatever you get, enjoy!