Unless you really desire a specific model, it's usually best to start with something simple.
Any Jeep is a great starter. They're inexpensive to buy, maintain and run. They don't have much power, nor top speed, but if you know how to dirve that's not really much of an issue for most off-roading you'd do with it anyway. Parts are eaiser to find for any Jeep than any other milveh. Many are made today.
The M37 is proabably what I'd recommend next. It's larger, has pick-up truck capabilities, and parts are available. You can get a diesel conversion for it, which really cures the fuel economy problem with most milveh's. The diesel actually is a better engine than the original gas one, but the gas one will due if you're a purist. Again, it's not fast, but they will go anywhere.
The commercial-based vehicles are a good choice for some. CUCV's, Dodge M880 series, and the Jeep M715 series. The CUCV's are the easiest, since they are newer and parts are more available. The Dodges are at the point of being old and worn out, and parts not really available. The Jeep trucks are old enough to have some aftermarket support, and many of the raggedy ones have already disappeared. The big advantage to these is the same reason the Army bought them. Logistics is easier. They use standard civillian tools, standard civillian parts, and behave in a mostly civillian way driving.
Once you get into the larger trucks and stuff, you are into needing special tools. Bigger support gear. Heavier and larger and more expensive parts to deal with. There can be DMV issues and insurance issues to deal with, though generally there's a way to get all that done. Because all of it is more complicated, I wouldn't recommend one for a first starter military vehicle. For what you've outlined as your needs, I'd stick with the lighter line.