1: Power delivery:
Much more linear in the GTR, almost feels like an N/A at times. Ofcourse, the power delivery is much more "push you back in your seat" as well, accelerates hard all the way.
2: 300kW?
If you're happy with 300kW, the cheapest turbo upgrade you can get are n1 turbos. Add in supporting mods, you're looking at about 5k. I was quoted this much from my mechanic, with a 10,000km warranty on the work.
3: In terms of dusty 32s there are a few outhere. Just get the car checked out by a mechanic with a brain and you'll be alright. Compression test, check for rust is a must. Try and find one that's already had a rebuild.
The Supra track is also a good track to go down. BUT, although you might get the power quicker than you would in a GTR, you won't get it to the ground and that's where the difference lies. The GTR will still be a quicker car. So it depends on what you like.
If you prefer RWD action, go the Supra.
If you prefer the AWD launch and quicker car, go the GTR.