I think you may have the wrong army to not buy too much.
Command squads with a standard to give a leadership boost, but I'm not sure I can recommend a commissar, although a lord commissar also give a leadership boost which can be useful if you really want one.
Infantry squads are great, but they can only take one special weapon and one heavy weapon team. I like the go big or go home and attach a lascannon.
The heavy weapons teams boxes can actually be used to make 3 heavy weapons (mortar, missile launcher and (heavybolter, lascannon, or autocannon) if you have 4 extra guardsman lying around.
Also as a beginner I highly recommend taking a standard battle cannon and heavy bolters on the tank since it has more shots and does a good amount of damage, a good jack of all trades.
Scout sentinels can be a great tool for deep strike denial with their scout move before the game since deep strikers cant deploy within 9 inches of an enemy.
I recommend getting lots of the 5 man boxes for 10 bucks a piece to fill out your ranks, and more battle tanks are a solid choice this edition.
Good luck!
Automatically Appended Next Post: I love using this site for tactics reference.
https://1d4chan.org/wiki/Warhammer_40,000/Tactics/Imperial_Guard(8E)
I also enjoy the youtube channel Mordian Glory.
https://www.youtube.com/channel/UCItUjGqA1uKs0l80L-VaAjQ
Also, since you said you are playing against space marines heavy bolters, autocannons, plasma guns, and even grenade launchers are cool for solid strength and some
AP.