All I'll say is cheap equipment usually = big repair jobs/bills. There is NOTHING cheap about equipment larger than lawn tractors. Auction prices will tempt you, but remember it's rare for somebody to send a perfectly good machine away to sale for no reason.
The other thing to watch out for is parts availability. There are numerous older tractors, both domestic and foreign, out there that are now comprised of unobtanium. With a PhD in cross referencing and fabrication, you can keep them going, but it won't be a task for the faint of heart.
Best bang for your buck in the heavy equipment world will be an old track loader. I'd only buy a CAT strictly because of the extensive parts support. A 955H or K can move some serious dirt for relatively small capital outlay. A smaller 933 would be good machine as well. A direct drive machine can most likely be had in operating condition for nearly scrap price vs more for a powershift due to ease of operation. There is more than a couple sentences to be said about evaluating the condition of one prior to purchase. If you base a purchase decision on grouser height alone, you're most likely going to be in for an expensive time doing repair work.
A skip loader like DN has is also a good choice for moving material, but it will most likely lack a PTO and standard 3pt.
To do any serious work in a pond, you either need a dragline/long stick excavator or to dewater and dry the bottom enough to get across it. A soft pond floor will swallow a piece of equipment in a matter of seconds. If the area is anything but bone dry, it's not wise to run anything on it unless you have a bigger piece of equipment on standby to pull the first piece out.