RS450 was never a proper model as such, but was a conversion done by Ralliart UK.
The first was an EVO 5 RS sprint, then subsequently I believe most were based on Evo 6 Extremes, themselves based on Evo 6 RS2s.
So, concentrating on the E6 variant - it has GSR gearing, no AYC or ABS, the uprated hosing and bushing of the extreme.
The RS450 conversion itself consists of Omega pistons, arrow rods, modified head, new cams, GEMS ECU, map sensor, hks air filter, exhaust and usually these cars have full AP brake conversions, as well as lowered springs. Car has Antilag courtesy of the GEMS.
The cars are tuned for torque. The RS450 was meant to run 450lb/ft of torque, though they never achieved that, and the gearbox couldn't take it if they did! It will actually have around 370lb/ft of torque and 350bhp. Some have more or less than this, but that would be an average figure.
A very quick car - and probably a good 2nd hand buy. Strap some decent suspension, and a bigger turbo and you would have a top EVO.
Having said that, it has probably been abused... and being a handbuilt engine, oil use will be high, and if it hasn't been maintained meticulously, it could be a bad buy.