If you can only afford one gauge, make it a pyrometer (aka EGT gauge). This is the only add-on gauge that will tell you when you are getting into the range of doing damage to your engine. Expect to spend $125 to $175 for the pyrometer, high temp Inconel thermocouple, and lead wires. The ISSPRO R607T which Jeff uses is a good choice, it is a mechanical gauge and only requires electricity for the gauge light, which can be swapped for a 24V bulb. However, there ARE 24V signal amplified electric pyrometers available as well, which MAY be cheaper, more accurate and more responsive that the R607T. When I get more info on them I'll post it here.
A boost gauge is "nice to have" but doesn't give you much information you can do much of anything about, unless you have a problem with your turbo. The turbo pressure is going to be whatever it is, and won't change unless you up your fuel delivery rate, and even then so what unless you get into the 20+ psi range, if which case you already have or are about to scrap your engine anyhow. The upside is they are pretty cool, run $35-$50 so they are pretty cheap and do give you some informative, if not necessarily usefull, information about what your tubo is doing.
Finally, if you are a gauge freak, consider a back pressure gauge and an inclinometer and gradeomter. The back pressure gauge is usefull if you are making exhaust system mods to quiet down that infamous Deuce exhaust noise without excessively restricting exhaust flow. They are in the $35-$50 range. An inclinometer/gradeometer is usefull if you do a lot of off-roading or just want to be able to calculate approximate rear wheel horsepower if you are modding you truck. They are also pretty cheap, $25-$35 bucks or so.
As far as I am concerned, Jeff has the ultimate gauge package. The only thing I'd add is a low air pressure light centered between and slightly above the pyrometer and boost gauges, and get rid of the annoying low air pressure buzzer. I'd include a low oil pressure and E-brake on source to the low air pressure light, and add an in-line 24V flasher to better get my attention. The low air pressure light will be pretty much in my normal driving line of sight anyhow, the flasher just insures a little more visable control to attract my attention.
I hope to have this project done in 2-3 weeks, I'll post pictures and details when the job is completed!