The only gauge that you really "need" is a boost gauge.
Depending on how far you plan on modifying your car, and what ECU you get, your gauge requirements will change.
The stock Oil Pressure gauge is pretty good (the sender can break, causing problems, but it's replaceable). The stock temp gauge is basically useless, I'd get an additional gauge for that if you don't go with the the Apexi PFC stand alone ECU.
Otherwise that's really all you "need".
If you get a PFC you can use the commander to display many of the things you're gunna want to keep an eye on. I have mine set to display four values: Water temp, Air Temp, Boost (it has a peak feature), and.... shoot, I can't remember what the last value is
(this will change depending on what I'm interested in watching on a day-to-day or minute-by-minute basis).
Personally I have a Boost gauge, Fuel Pressure gauge, WB afr gauge, and (soon to have) a flow meter for my A/I system. These are in addition to what I have my PFC displaying.
Some could say that I'm running overkill, but I like the peace of mind in being able to monitor just about everything that's going on at a glance.