The TS has max hp of 165 at the crank.
The rear drum to disc swap has been done many times and written up extensively in the forums. A search will give you loads of advice and some sites with pictures and instructions.
The rear discs from early legacies, current legacies, RSs and WRX all fit. If you have ABS then you should get the brakes from an ABS equipped donor car to keep the sensor mounts. Junk yards or people parting out wrecks on the classified forum are your cheapest sources. You will need the knuckes including hubs, calipers, rotors, pads and ebrake cable.
IMHO this mod should be fairly low on your list after you have addressed tires, wheels, suspension, and front brakes. The parts can be fairly pricey depending on your source and requires a bit of skill to do.