Usually when you turbo your car you want to get a good engine management system, a boost controller, new clutch and if you're going for big power gains you should replace your vital engine internals like pistons and transmission. On a stock tranmission you can run about 10 psi of boost (usually) before you start destroying stuff. For example, the SCC project Impreza destroyed a half dog gear box because it had so much torque. Also you might want to reconsider getting the biggest turbo you can find because issues such as turbo lag can arise. Larger turbos take more energy to spool up to full boost. Also a large turbo will cause you to have to change the compression ratio of your pistons if you want to safely run your car without piston knock or detonation. My advice would be to start off with a simple kit that isn't big and keep the boost under 10 psi. This way you can retain your stock transmission. I would also recommend a 3 inch cat back system and a new clutch to realize the full potential of your turbo system.