Larry950,
The guys are right about the tractor. I know, cuz I tried it myself. It takes a loooong time to dig a hole big enough for a foundation. Personally, I would rent a excavator for a week or so if you feel you can pull it off, and save yourself some large $$. A JD120 is a big machine, but no more difficult than a smaller one to operate. I'm just about to rent a JD80 which is about 16,000lbs. My soils are easy to dig gravel and it should do just fine. If you have lots of large rocks, I'd say go with the 120 for a few more dollars. --Just make sure you do some research on site layout before you start to dig and potentially screw things up. It's not rocket science, but you do need to do some reading and talking to folks who have done it themselves before you start. Good Luck.
Jim