I definitely agree with Terry. It's not such a great investment converting it from N/A to Turbo. A friend of mine had done exactly the same thing. It costed heaps, spent a long time in the workshop and after all that, he got a sound of a blow off valve along with countless problems. So in my opinion it's not an option I would take. But everyone has their own.
I personally love all R34's. My friends car was actually my old one. It was super reliable, easy to drive and also extremely comfortable. I bought mine back in 2005 with less than 30,000kms, completely standard and I never had any problems with it. I only sold it to buy a GT-T.
Things to check for as Terry said is check for signs of rust, cracks around the door window seal, camber wear on tyres and pretty much all your other obvious visual inspections (damages, dents, scrapes etc)