for batteries there are several schools of thought.
Some people are purists and will insist that you require the 6T series batteries for the truck. The flooded version only has a 6 month specialty/commercial warranty and are several hundred dollars. Most auto parts stores don't keep them in stock. The AGM version will likely have to be ordered from a manufacturer to get a decent warranty, but you're talking probably minimum $600 per battery.
The 6T series is overkill for the HMMWV application, but the military finds advantage in standardizing batteries across the fleet, so it makes sense for them to use it due to logistics. In reality in a warmer state you can start a HMMWV on a pair of lawnmower batteries if everything is in good shape (not recommended though)
Group 24 batteries fit pretty well, but be sure to secure them tight as if they shift they can short out against the hold down if you have the older alumium type. You do need to make sure you purchase the flat top version for the bracket to work without modifications.
If you're staying in cell service and not doing any serious off roading then regular flooded batteries will be fine. As others have mentioned, they are cheap and readily available. Getting the maintenance free version is better as you don't have to go check under the seat and refill them with distilled water like standard flooded lead acid batteries are supposed to have done regularly.
If you do a lot of off roading or away from cell service then AGM batteries are a good choice. They do cost more, but last about 30-50% longer life than flooded cell batteries, and stand up to vibration and rough handling better than flooded.
Picture for example of what a pair of group 24 AGM batteries look like installed and secured. I mostly went AGM for the reasons listed above; I take my truck off road onto the jeep trails and minor rock crawling, as well as it being my bad weather work truck and occasional remote camping vehicle. If it was primarily a pavement princess I could have saved a lot of money (these batteries were $300 each at the time when the cheap wally world batteries were around $60)
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