I haven't seen you post your questions for the Growler owners, so I'll take a quick shot at them from my perspective. You have many nested questions, but I'll try to touch all of them, and just from my perspective.
The private sales are tough to assess and vary widely. Original auctions ranged from about $5k (empty shell) to almost $60k (freshly upgraded and rebuilt by Uncle Sam). As a general rule-of-thumb, most Itvees appear to bring 2 to 3 times their auction price. This is entirely expected and valid for relatively rare and unique vehicles like these, as described in this post
. One thing I've found, is that there is no reliable "average condition" for ITVs, as there are so many variables. It's worth what the buyer and seller can agree it's worth on that day.
One of the most attractive features of Itvees is their design requirements. This included every part possible be commercial off-the-shelf (COTS), and one reason I've made one of mine a nearly daily-driver. With the exception of the chassis structure and body parts, most every part is available online with a click. Not to say they are cheap, as top-notch parts were used throughout, but they are mostly available parts or have alternatives. While "rare" usually mans hard-to-find parts, this one is the opposite for most things.
For the engine, MWM is a global producer, just not for the American market with it's much more stringent regulations. All who have tried to source engine parts for the Navistar/MWM diesel (that I know of) have been successful, mostly from European sources where it has been used globally in Volkswagen trucks. Other sources are countries that had them in Nissan, General Motors, a few Fords or other users. MWM has almost 1/3 of the diesel market share globally (they're big), but we Americans don't see that.
The effective parts difference for owners is that Hummvee parts have specialty dealers, whereas Itvee parts are available to everyone, so the convenience of ordering from your favorite supplier is not a common option. Dealers can't mark-up a master cylinder for $300 when they know you can buy it at NAPA for $60. Instead, you have to determine what the part is and where to find it yourself, often from your local auto parts store or online for a lot less than other MVs.
Other times it is a bit confusing or frustrating, because you know the part is out there (maybe everywhere) and you just don't know what it is or who has it. It took a couple months to finally confirm the control arm bushings were a common Chevy part and $5 each from RockAuto, or that the axle shafts are used by the Porshe crowd. Enter the power of the Growler community and sharing knowledge. Treat it like servicing a custom-built off-road rig, and go at it.
The 600 page operators manual is available, but the service or shop manual equivalent is still classified as a research program and part of a "weapons system". This is a big hurdle, as you don't have much specific info to tell you how to work on it or where wires go. Again, the community has done some very intensive research and fact-sharing to ease this situation. Still, you would greatly benefit from some mechanical and electrical skills if you need to do miantenance and diagnostic work. Most things are familiar to mechanics, such as the Bosch fuel injection, used on millions of engines from Cummins to your neighbor's VW, or the Chevy brake hydraulics, Ford differentials, GM Blazer/Colorado transmission, NP241 transfer case, Fox shocks, Wilwood brakes, etc.
Condition is relative, and the same process for any MV. First is to remember that unless the seller did a lot of work, you found a vehicle sitting for 10 to 15 years. It will need extensive maintenance, same as any "barn find" of any type, even with easy to find parts. Belt, hoses, filters, fluids, wipers, etc. Priority is to verify every last thing and function works, properly. Check everything. Anything that is missing, fritzing or broken can mean added costs, and especially added labor. Many are missing cannibalized parts, and while you can probably find new ones, it can be a pain to locate some stuff, and pricey to install unless you're DIY. Most of them have bent skid plates, and often bent driveshafts from stupid forklifting.
Likewise, extra stuff is a bonus. Your winch and its mount adds a chunk of value (if it works), although even that is available from Ramsey new (24V Patriot 9500), and the mount is not exactly rocket science to make yourself. Some canvas is reproduced, but doors not yet, so unique body stuff like that is a nice plus.
Yes, and I saw one original early-on auction. However, most now would be fabricated reproductions. Search for threads about parts, plans and 'complete' setups from other belters.
Sure, I have some random ones: The soft tops and doors will not keep you dry. If you are tall, you won't fit into the short PMs with less leg room. Maintenance can be a bigger challenge if you're used to a Willys, as there is a lot of hardware crammed into these things, and much is not easy to get to. With the turbo-diesel clattering, the compressor humming and cooling fans whirring, they can be annoyingly noisy. On the other hand, with their pneumatic sneezing and farting as tires auto-adjust pressure, or suspension operates, they do exhibit unique character.
I have only had one of mine out 4-wheeling once, so I can't help much with that perspective, although others can that do. It was fun offsetting the suspension (leaning) when cross-traversing a slope, that nobody else could, and tight maneuvering with 4-wheel steering was awesome. They were highly praised by operators for their impressive performance during their testing and trials. They aren't a "jump into of holes and over moguls" type like a Polaris or quad, and don't take corners at high speeds at max ride height... LOL There are reasons Marines received extensive driving instruction, and the dash-mount Lev-O-Gage (tilt angle) they have can be important. I imagine they would be very happy pulling your camping trailer into the outback.
I love my stubby PMs. They are like a heavy and fast over-powered Samurai. They get more attention than most hot rods and collector cars on cruises. Waves, salutes and honks are endless, but interruptions and questions happen every trip out. That's great for a while, but now I carry info sheets to hand-out following the 150th time I heard "what the hell is that?". Crabbing down the street with cross-controlled 4-wheel steering is a huge crowd pleaser, especially after hissing from max suspension height like a scared cat to squatting on its haunches. Get used to cameras and videos on cruise night, while gawkers ignore the Ferraris and Lambo's around you. Seriously. I had mine a month and a paparazzi-style shot appeared as the sole photo on the front page of the newspaper covering Independence Day stuff. I've owned multiple classics, and this one is a magnet.
I hope this short overview helps, and the Growler community is very friendly if you want more answers. I wish you well on your decision!