The most important gauge to add when upgrading is the Pyrometer.  The Pyro, as it is called, will provide you with instant exhaust gas temperatures, also known as EGTs. EGT is the temperature of the engine's exhaust as it enters the turbocharger.  When these temperatures get over 1300 degrees, they become damaging to the engine and turbocharger.

The pyrometer is also one of the first indicators of trouble in the engine.  If an injector clogs, or has a problem, a pyrometer will note the change. 

By monitoring your exhaust temperature, you can detect changes in the exhaust temperature which may be early signs of trouble, and you will know when to back-off the throttle or downshift to keep the temperatures from reaching damaging levels.

A stock engine by itself will not produce damaging EGTs, but when you increase its power output (via chips, or otherwise), the EGTs will also rise.  A pyrometer will then become necessary to keep track of them.