Although the Unimog is a nice utility vehicle you would want an older one with fully mechanical fuel injection and plenty of spare parts such as brake components, suspension parts and driveline parts if you are in North America.
You're going to want all of that for any vehicle anyway. After a few years of sitting in junkyards or on the side of the road, most of that is useless anyway due to rusting. And yeah, I'm not talking about fancy pants new 'mogs, I'm talking about classics like the 404.
If you really want bulletproof get an old Dodge Ram diesel. The older 6BT series Cummins had fully mechanical injection which means even if the battery is dead and the alternator doesn't work, the engine will still happily keep running. Mechanical injection also means if you have a dead battery you can pull or push start the engine.
Ah, but only the (IIRC) 1994-1998 (might be off a year on one or both ends) has a decent injection pump with one pump per cylinder. That's another advantage of the particular bosch jetronic system used on the OM617; it's not unique but it is unusual because it makes for an expensive injection pump and expensive rebuilds. The OM617 has only five cylinders, so it was less additional expense. If you've got an IP with two pumps and a rotor that's more or less three cylinders already, why not get rid of that rotor and add a couple more pumps?
IPs with less than one pump per cylinder wear out faster. And in particular, the diesel chevys and early diesel fords use the Stanadyne DB-2, which is crap. Pretty much guaranteed to go out around 200k on modern fuel, probably faster on WMO or WVO.
Manual vs automatic transmission is debatable depending on the engine fuel type and years made.
Not really. I like driving automatics just fine, but they are more prone to failure, period the end. In particular, they will not work in the event of complete electrical failure, which again is a non-negotiable point. Even if you have a gasser, you still want to be able to bump start for those times when your battery won't run the starter but will run the ignition.
If you are looking for an apocalypse ready vehicle you are better off building your own.
At least with a 'mog you have a chance of locating a donor vehicle.
Everything sold today is so full of electronic junk and emissions systems that they are unreliable without a tow truck and garage on hand.
And that's why the proper vehicle is something very old. I've fantasized about putting my OM617 into an old F100 and putting a duramax or TDI into my 300SD. Then I could get rid of my F250 with its ridiculous 7.3. I just don't need this much engine, though it's fun to be able to pass the land rover that's been fucking with me uphill on the right while hauling a load of firewood, it's not necessary.