One thing about the PLX gauges, if you're going to monitor fuel pressure stay away from them because their current sensor will corrode. They are working on a suitable replacement though.
I have fuel pressure, water temp, oil temp, oil pressure, boost and dual EGT.
Fuel pressure is good for troubleshooting if you're having trouble starting the car. That way you don't have to crack a line to see if you have fuel. Also just good to monitor in general. If you get an aftermarket FPR you can get a little gauge on it and that would help with troubleshooting but not so much constant monitoring.
Water temp is IMPERATIVE for a rotary, so definitely get that one.
Oil temp I don't personally believe is as useful as an oil pressure gauge, but it looks like some here disagree. My thinking is that hot oil is better than no oil, and hot oil wouldn't hurt your engine as bad as hot coolant/water. So that's why between the two I'd go for oil pressure instead, but I got my oil temp just to see what dual oil coolers could get the temp down to.
I hardly ever look at my boost gauge because my boost controller has those readings and it also holds the highest boost you reach until you turn off the ignition or reset it, so I pay more attention to that than the boost gauge. I'm getting rid of all my gauges and going to SPA dual gauges so I can simplify the interior and not having it looking like a space shuttle cockpit.
Dual EGT are good to monitor, mostly for tuning, but that would be after quite a bit more modding. I don't think you really need one or both if you're mostly stock.
So I'd say if you're mostly stock, I personally would get water temp, oil pressure and maybe even fuel pressure too.
And here is a great site for blinker fluid, muffler bearings, etc...
http://www.kalecoauto.com/