Howdy! From what I have read here and a little bit of my own intuition, I would say spring for the big excavator. I worked on digging out part of a pond with a backhoe attachment on the back of a tractor and it was VERY ineffective. Those buckets on the mini-excavators can carry maybe one or two cubic feet of dirt. Full sized excavators are usually around 20-30 times larger depending on what you get. The big one might cost 4 times as much, but you will be working 20 times faster in essence.

As far as the DIY aspect goes, call Paul Dorsett (979) 279-2946. He is in the Pond Boss Resource Guide. I have met him, know people who have used him, and everything is positive. It will be more expensive than doing it yourself, but his experience will be invaluable. If it was me, I would at least bring him in to consult with you.

Definitely take a peek at the soil survey for your area. I know a few guys around Caldwell that have leaky ponds due to not having enough clay.

Water dries, rocks crumble, and trees die. The only thing that is eternal is the reputation we leave behind.
- Ancient Viking Proverb