You have the codex, so you're a step ahead of the usual. Start with 2 boxes of Cadians, and a Chimera. A Weapon Team box would be a good idea too. This will get you the minimal 1 HQ and 2 Troops necessary to play most games and is a good starting point for a Guard army.
JO w/ 2 Grenade Launchers, Mortar (or whatever conversions make you giddy)
Jo w/ Heavy Bolter, 2 Grenade Launchers
- Squad w/ Missile Launcher, Grenade Launcher
- Squad w/ Lascannon, Grenade Launcher
Fist w/ Autocannon, Grenade Launcher
- Chimera w/ Multilaser, Heavy Bolter
A little bit of sprue cutting will be needed to make an additional tripod or two, but one box of heavy weapons can outfit a small (very small) army of roughly 500pts. You may have a few points to spare.
Or, if using doctrines, An Command Squad box and a box of Kasrkin will get you a complete army, though it really doesn't work well (or look too good) at such small levels.
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