1. Boost refers to the forcing of air into the engine by means of a turbo... damn that definition sounded good Basically boost is the pressure at which the turbo is forcing air into the engine. When people say that they boost their car, it means that they are driving it within the boost range. Driving off boost means the turbo is not spooling and the engine is getting air at atmospheric pressure as apposed to boost pressure.
2. R34 GTT's were never sold in Australia, so if thats the price you're looking for, its not possible. If it was, I'd say it would go for a bit more than the current V35. The only reason I say more is cause of the lack of turbo in the V35. So say $60-70k. Some one can probably estimate better than me.
3. Performance is a very broad term. Supras would probably be similar overall, but you need to be more specific. You could mean track performance, street performace etc etc
Hope this answers your questions