I have been driving around in a rex for the last few years and decided to upgraded to a sprinter.
Doode ... are u for real? You must really believe in Initial D to think that a Rex is a downgrade from a Sprinter then ... even Bunta beats Takumi in a Sprinter with his WRX Sti ...
Guess you're already living up to your name ...
Anyway, the T-50 gearbox should withstand most power unless you run something like a 4AGZE. Sprinters are light cars and their chassis were never meant to take much power ... they are about balance and not power.
Still, with the money saved over from trading in the Rex, you could go either a 20 valve or 4AGZE conversion. If you wanted something more, then a SR20DET or even a Rotary conversion ...