Im currently making 150rwkw with a slightly bigger turbo (R34), 10psi, intercooler and zorst with my GTS4. Not bad considering I have a very lazy fuel pump. (Same dyno, same night a near stock twin turbo 300zx made 139rwkw to give you a reference). All you have to do is pull the fuse to make it a RWD car.
Common problems
Hicas 4WS
AFM
Can have idleing problems mainly due to the AFM
Lazy fuel pumps
Stock suspension is starting to wear as they are over 10 years old now.
GTS4's are heavy but are great with the 4WD. It can get you out of some pretty messy situations. GTSt's are really nimble and quick. Have a look at the interior, does it look like it has done 70ks? Generally most imports have had the odemeter wound back a few thousand kms, but interior condition is a good way to guage. Also look for a sticker on the timing belt cover to see if it has been changed, then you know it has done more than 100,000kms.
Apart from that a very enjoyable car to drive and very easy to extract power out of. It is more of a sports car than the R33 mainly due to its hard edged look and aggresive interior. It does lack a little bit of torque down below but makes up for it high in the rev range.
Good luck with the purchase.
Doc