My dad bought a 6 x 6 back in 94 I believe. He also bought some plastic tracks for it and had a custom made canvas canopy and window. I used it quite a bit, we pulled a few moose with it. He sold it 2 years ago and got $3200 I believe, but he had well over 13k into it.
I think the new ones are better, and the older ones had more problems. I think my dads had an 18 HP Briggs and Stratton in it. It also had the hydraulic steering, which I heard was much better than the mechanical steering models.
The biggest thing about owning an Argo is maintenance. Greasing and oiling every time out will extend the life of your unit. I don't think he ever replaced a chain or bearing in his, but we always greased and oiled it every time out. That Argo went places that a quad or side by side would never go, no question. We took a moose out of the Little Smokey Goose River country one time, 3 of us in the Argo and pulling a 4 year old bull out, it was a fairly long haul and it was sleeting out, it was a slow pull but it got the job done.
An Argo is a love hate thing. When they are working good they are an amazing machine. Getting them stuck is no fun and like any ATV you should install a winch.
I know they promote the thing as being great in water, and to some extent it is, we crossed deep creeks and beaver dams, but as far as taking it in a bigger body of water I would avoid it. The older units had an issue with pushing the drain plugs up and into the sump, sometimes you wouldn't realize it until crossing a creek and taking on a bunch of water, I imagine the drain system has been changed on the newer units as I felt it was really a design flaw.
What I would recommend before buying one is going out in one to see how you like it.