To be perfectly honest, I'd pose the same questions over at Subaru Impreza GC8 & RS Forum: RS25.com or the NA section of NASIOC. Subaru turbo drivetrains are very different from the NA drivetrains. Regardless, I would have a leakdown/compression test performed on the vehicle to investigate the health of the motor internals. I have heard that the early NA 2.5L motors had head-gasket issues, so that's good that the part was recently replaced. I would stay away from a vehicle with modifications, though I'm sure it's much easier to find a stock RS than a stock WRX/STi.
Even the turbo drivetrains have a lot of life left in them at 150K miles if well-cared for. I don't see why the NA platform would be any less reliable.