Matt; With what you provided in the way of info this will be very general. Find out first if it is a series 1,2, or3. is it gas or Diesel. these things DO make a difference. Generally speaking the D-21's are very good all round tractors. Powerful for their size and wieght, HOWEVER, make absolutely certain the power director clutch, which runs the live hyd., and pto is in good working order as these are VERY expensive to replace. Not to mention a big job. rated at 77 to 83 horse power, depending on the series, they are for all intents and purposes fairly stout pullers.
They have extremely tough transmission - differencials and are not prone to break down. The only exception is the series 3 with the perkins diesel, Some have a problem with blowing head gaskets. Aside from that I have had very few problems with any I have owned. The cab is something strange, It must be a adaptation or kit cab, I'm not aware of a fatory cab for the D-21 unless it is something offered late in the production as an option. Hope this helps.