A couple of things come to mind for your research. All military-developed equipment is built to standards that satisfy a tactical mission requirement. Because of that (and sometimes politics), you will find some expensive parts and capabilities never even considered in civilian use. Which may or may not be expensive to keep up.
As I remember (which is more problematic with time), the engine and transmission are GM: think they put the older 6.2 - 6.5 liter diesel engine up front (which is probably fine, but it did not fare as well for farmers' pick-ups). The old and reliable Turbo Hydramatic 400 was the transmission.
The condition and use of these machines is important. They were routinely run heavy and hot in tactical environments, which necessitated larger versions of the engine and turbocharging. (read sandbags, armor, heat, and heavy). Coolers helped, but many mods were made to try to keep up. All that means is be careful which one you're looking at and where it was in service. Watch for salt water corrosion also.
Additionally, which version are you looking at? The services have all sorts of mods to the HMMWV, depending on end use. Some can mean problems and increased costs to repair.
A suggestion: look up the "Steel Soldiers" website. Think you might find lots of folks there who own, operate, and repair this end item. And some good TM sources.
In my opinion, naturally. Best of luck.
