From the mid - late 1970's until the early part of this decade there was a Deutz/Hesston, then later Agco, dealer in our county that did very well and sold a lot of air cooled Deutz and Agco tractors. I've seen them used in some of the toughest imaginable conditions - chopping corn and hay, plowing, dairying, etc, and they've held up well, don't seem to have any problems not associated with other brands, and appear to have been sound values. As with any used tractor purchase, I'd do some investigating into the machine's usage and background.
But, IMO, the pedigree is sound.