A few boost questions about mk3 mondeo

[Dave5]

So I have a 2002 mk3 mondeo tddi with fixed vane turbo. Engine part is still more or less stock ( had the dmf to smf conversion done)
I have read some different forums/posts and come to a conclusion that the car could problably use a bit more boost/power because well..boost ! :)

The easiest (cheapest) would be to install a boost gauge and a manual boost control valve. 

My questions at this point are. 
1. How much boost does the stock setup produce 
2. How much can the stock piping take
3. When will the ECU understand that i am messing about with it and go into fault
4. At what point do i risk runing it too lean. So that the injectors, pump fail to come up with enough fuel. 

If i have overlooked a topic that covers these questions already, then feel free to post a link and point me in the right direction. 

Thank you