I will second Butch(OH) in that you need to make sure the engine is 100% as they are expensive to repair and some parts are just about impossible to find. The diesels where prone to cylinder head issues. If you do buy it make sure and NEVER over heat it as that usually ruins the cylinder head.
Truthfully if your going to be just using a bushog and running a post hole auger you do not need as big a tractor as a AC D-19. Also the concern I have is IF you have an engine issue down the road your going to be hurting on finding parts and the cost will be high. You can easily put more in the engine than you gave for the tractor. IF your set on AC then I would look for a D-17 as it would be large enough for your needs and the engine parts are much easier to find. If brand is not important than I would look for a Ford 860 or 861.They can be bought for about what this D-19 is being sold for. They have a factory three point and parts are readily available from many sources.
So the D-19 is a fairly good tractor but parts availability can be an issue. Also the snap coupler three point conversion works but it is not the same as a factory three point. It is "different" to use. The three point/snap coupler control is different than the common three point quadrant on most factory three point tractors.