The 24v engines have an electric lift pump that draws fuel from the tank then pushes it through the fuel filter, then to the high pressure injector pump (VP44). The lift pump is notorious for failing on these engines. Typically, you'll see 10-15psi after the filter (depending on engine load) with a good pump.
The injector pump needs at least about 6psi (that number is debateable, but close) to keep it healthy. Most of the fuel supplied by the lift pump just passes through the VP44 and is used for cooling, and only a small percentage is actually pushed into the engine. Without the surplus fuel, the VP44 could overheat and fail ($1,200ish for the parts).
The typical mode of failure is for the lift pump to fail (which is a $140 part) followed shortly after by the demise of the VP44 due to the lack of fuel pressure. The engine may still run with a bad lift pump, as long as the prime isn't lost (like when a fuel filter is changed). The FP gauge will tell you if you have a loss in supply pressure before the expensive pump quits. I put mine on at about 40,000 miles and found out that my pump was weak. I'm currently on my 2nd in 72,000 miles. The fuel pressure gauge is probably as critical for these engines as a pyrometer is. It's also a handy way to monitor your fuel filter's health.
As far as the rest of the gauges, pick your favorite look and go for it! I've got Isspro EVs for my pyro and boost (great match for factory look) and a SPA combo for fuel pressure/oil temp (could be used for trans temp in an auto). I prefer needle gauges for rapidly changing values, since they also show rate of change better than the digitals do. I like the SPA for the FP/Temp 'cause I can program the flashing warning lights to go off if pressure drops to low or the temp gets to high. I got mine from Doug at smokindiesel.com and got a really nice package deal, and he even painted the a-pillar cover to match my interior.
Attention: TDR Forum Junkies 
Is the feul pressure necessary? Is it just to let me know of any impending failure of the injection pump? What about a boost gauge? If so which one? The pyrometer seems like a given to monitor the exhaust and intake changes and their effects. Thanks in advance.
