hey mate have a look here, all the info you want www.r31skylineclub.com
There are two types of R31- the local and the Import.
The local is from 86-90, series 1/2 from 86-88 (rectangle taillights and square frount end) and 88-90 was series 3 ('hotplate' tail lights and newer curved frount end.) They all had Rb30s (pintara, which has the same body as a r31 has the Ca20). Manuals are easy enough to find, but autos are really common. Every Rb engine fits in relatively easy, and Ive seen Cas go in, but not SRs, but I dont think it would be that hard. Youd better off putting a Rb25 twin cam head on a rb30 bottom end (which is done by heaps).
There is GX (econo bog pack) GXE (better) executive (s3 only, repmobile) Silo (sports model, has LSD) and TI (luxury model, auto only). Theres also the GTS models, the s1 ones are white, the s3 ones are red. only 200 of each and they came with lots of extra goodies. Highly desirable cars by r31 lovers, but arent even noticed by most.
Imports came in Coupe form as GTSs, GTSXs, and GTSRs (verey rare). Had the old redtop rb20 turbo. there is also the 4 door one, either the GTS or the passage which is the luxury one. It is pillarless, which is a very cool feature.
8k will get you pretty much any R31 you want, even a local spec GTS (rare). Id get a Import for that money, cause if you modded a GTS every R31 lover would eat you. YOu could also pick up a local spec GX and mod the shit out of it