1. you may want to look at a full race spec decat, there fairly quiet when just tootling around, have a nice bark when you give it right foot, yet not restrictive on performance.
2. you don't need any external boost control, that's what the ECU is for ;) If you want to up the boost then remap it or your asking for an engine build bill.
3. second hand turbo's arn't big money and are typicly bolt on and work, if the turbo's not right for the engine it wont work and be laggy (although could work very well, untill you fit it and have it mapped you wont know either way). New turbo means full remap or your asking for trouble and engine build bill.