Do you have any kind of tractor that can pull/push the dirt away from where you will be digging? An other option is a dump truck/trailer? With out that, you will be slowed down considerably. I am guessing that with what you are showing, you probably wont be able to reach out and down from the edge to the center of the hole and get the dirt out of your way in one pass. That means double handling with the bucket and then you still need to move it to where you want it spread out.
I am not a heavy equipment operator, however I have done various projects myself. I would look into a dozer like mentioned earlier.
However, that could be tricky with no experience, you might have pretty steep sides if you want to get any depth to a 50' wide pond.
Also, there will be a good chance you'll need a trash pump, and be delayed with rain the longer the project takes. Once you start digging, you want to stay on it till its done, at least that would be the best plan of attack.
You can rent bigger excavators, but not dump trucks, at least that's what I found. You can hire a dump truck.
