155 on a stock turbo?? That wont last very long.......
Elaborate please.
How big is your budget?? As that will determine how far you can go......anythings possible with money!!
I wouldn't say I'm ready to fork out thousands straight away but could invest some money on regular basis.
Your peak BHP is achieved at approx 4200rpm; for the turbo to maintain that it will be working very hard. The harder it works, the quicker it will fail.
Equally you need to remember that its not all about peak BHP; its about how the power is developed and delivered. You want a car with a wide powerband as that makes for a quicker and more driveable car than one with a narrow powerband and peaky BHP
I'd collect the parts you need and fit them in one go as that way you'll save on labour costs (if not doing it yourself)