Most people will say that you need a boost gauge and a fuel pressure gauge. I do not agree. Yes, it is nice to know your boost and it is nice to see that you get the right fuel pressure, but it is not necessary. If you only want 1 gauge then I would go with a wideband AF gauge and invest in a datalogging device that will log your fuel pressure and boost levels.
Now I went the other route and got all 3 gauges and a datalogger. It is more for appearence reasons than performance reasons.
Check the link above for the instrument clusters etc. They are good.
Do not buy the KB gauges. They look cheap. Go with a set of autometer gauges if you want the boost & FP. Do NOT get an AF gauge unless it is a wideband... they are only good for a neat light show.
Joe