at the very least i would say boost and fuel pressure gauges. if you can afford it also get an air/fuel setup, as it will really help you set the right fuel pressure. i run all the above plus an oil pressure and water temp. when you start toying with your car and beatin' it around its always a good idea to monitor all it's vitals. good gauges and monitoring can help avoid catastrophic events and aid in tuning. hope this helps
if it isnt a wide band o2 it isnt worth crap..........they have came down in price and are worth the ?????? i would have to agree on boost,FP and oil pressure cause if it starts to flex due to the new higher demand for pressure you will know before it quits on you.....