Try and stay away from the '02-'03 QR25 motors. The laters years seems to have improved, but not as much. They're known to have pre-cat failures. They seems to build up a rich air fuel mixture inside the cat rather than in the cylinders causing bits of pieces of the cat to be sucked back into the engine. If you do purchase it... Install new headers, and loctite the butterfly screws.
I'm a member of b15sentra.net, a very good place for information about the B15s. By the way, I own a '05 B15 with the QG18 motor. Not performance machine, but very reliable compared to the QRs.
B15 Sentra . Net