My suggestions, in no particular order:
1. Boost/vacuum (vacuum is just as important as boost for diag)
2. Wideband O2..Narrowband is useless on a boosted car.
3. Oil temp
4. Fuel pressure (if you need a forth)
5. Mod the stock water temp gauge to read actual temps (easy if you can solder...otherwise I'd be happy to do it for you)
I hate gauges all over the car, so this is what I have:
1. Boost/vac gauge molded into steering column
2. WBO2 in rearview mirror (and only have because I was in the mood to solder stuff...I haven't had a WBO2 in the car for 5 boosted years without a single issue)
3. Oil temp in vent on left side of steering wheel (need to move - I hate it there, but I was in a hurry to mount it to leave FL)
4. Modded stock water temp gauge