First time dozer purchaser (no snickering please)
No snickering allowed Mbdog, this is a grown-up forum. Everyone here has a first experience, be it operating and/or purchasing their first machine.
One thing to consider when using a 6 way to clear trees, you need to know their weaknesses. Don't use the ends of the blade to push or pry on trees or stumps. If you are pushing larger trees, make sure the tree is centered on the blade. This allows the stress put on the blade to be transmitted to the trunion and thus to the frame of the machine.
As far as purchasing a used machine, you can't check it out enough. Ditto on taking oil samples and having them sent to the lab. The results will give you a good picture of the state of the components. Check the undercarriage throughly and you can also get the local Cat dealer to send a product support rep to measure your undercarriage to determine the life left. (There will be a cost involved if you don't have a relationship with the dealer but money well spent IMO)
Repairs are expensive on any piece of yellow iron, do your homework up-front. A cheap purchase price can turn into a money pit quickly.
Good luck!:drinkup