Gauges aren't a bad idea for a tuned truck...
About $130 each, on average, and a pillar pod is about $60 iirc
Have you installed a lift pump yet...?
It's a bit more than you're looking to spend, but one of the most important mods I can think of when starting out, preserve those injectors and CP3:thumb:
Lift pumps go for about $550+
Then a full EGR delete is a good next step if you don't have to worry about emissions, a blocker plate is effective for guys needing to switch between being emissions compliant and hot rodder friendly.
EGR delete kits vary greatly in cost, anywhere from 350- over $1G, blocker plates are like $35
A TransGo Jr valve body kit is a really good idea for a tuned LBZ, they're pretty cheap, make the transmission shift better, IMHO...and add a little bit added security against clutch failure, a little bit...
TransGo Jr kits can be had for 100-150
I'm a fan of BT Dieselworks (Ben Tyler) lock up box....
Icr how much...I wanna say $550
Getting started on the front end parts isn't a bad idea...tie rods, idler pivot assembly, cognito braces (some hate em, some swear by em) or a center link brace
You could get started on a good set of shocks, like rancho 9000 adjustable, or QA1 adjustable shocks
275-450(set of two)
There's a ton you can do for a 300-700 price point....you gotta figure out what's most important, I think a lift pump is pretty important