Something Steve touched on but didn't elaborate on with old dozers; what are your mechanical skills? If you can find a "turn key" machine, it's going to need maintenance along the way. Fuel, oil, oil filter(s) and air filters need changed and cleaned regularly not to mention greasing the machine. These old machines are not like modern cars that need virtually no maintenance, but then most of this modern stuff won't last 70 years or so either.