Case dozers of that size are good machines, pretty simple to operate and fix and case was a forerunner of the two speed shifting, or speeding up and slowing down a track to turn instead of just stopping a track to turn. I'd guess it has a six way blade on it, most did anyhow, and was a great way to go with a dozer that size, very useful and made them an all around purpose built machine. For 12k, thats pretty cheap, for my area anyhow, just check it over and maybe take some oil samples of the components to see if they are in good condition. As for the engine, if nobody has changed it out it should be a cummins 4 banger in the d model, I've got a C model with the older case engine in it. As for what else to look for, not much just run the dozer for a few hours and see what it does under a load.