First thing i would do is give it a bath and polish
Having just been though a bare metal resparay, its about how much you want to spend. Lots of shops around that can do a great job but for around 10k-15k price range
As has been said S20 is wasted on copys, L31 is what im going for down the track stick with carbys too.
SWR will do all the engine and diff work like most specialized shops be prepaired to pay. There are number of good shops around that could take it on, maybe give us am area of sydney to give you the close's, but the likes of UAS, CRD, Advan racing would have no problems as well.
Looks like a nice starting point, should be good old school skyline meets next year LOL
Brad