Brian has it right regarding the glow plugs - they help heat the air in a cold engine to aid in starting and cold idling. Isuzu uses an electronic timer/controller that turns the plugs on and off and also controls the voltage delivery to the plugs (low or high). All of this depends on the coolant temperature. There is a sensor in the water neck that the controller monitors. There are also a couple of relays and a large resistor. It is worth mentioning that there are at least 2 different systems I've seen schematics for in Isuzu manuals. I've adapted the older system from a 2.2l diesel for use in my project. You may have luck finding the later system. I think the later version also controls EGR.

As far as engine monitoring goes, I am using the stock gauges that came with the Trooper (with the exception of the tachometer - that must be diesel-specific, and I replaced mine) to monitor coolant temp and oil pressure. The diesel engine senders work with the gas gauges.

I don't think you need to monitor exhaust temp or go for any more ambitious gauges etc. unless you are planning on modifying the engine (increased boost pressure and fuel delivery). Generally there are only concerns when the engine's power output has been increased. Just follow normal maintenance procedures and you should be fine.