I'm wondering how the things keeps water in it, if only part has a liner and it's sandy soil.
Does the spring feed a stream that fills the pond, or does the water gurgle up from somewhere in the pond? If it's the latter, you'll have a heckuva time trying to excavate the sand without the walls of the excavation caving in on you. You may need to do some involved de-watering before you can excavate.
Placing the stones is easy enough - get an excavator with the capacity to lift the stones you'll place, then just use straps rated to handle that load. Sounds hard but it really isn't.
As for how much stone, that's a simple equation. First, you need to find out the density of your stone in weight per volume (150 pounds per cubic foot, for example). You've already mentioned how big the average stone is - turn those measurements into measurements for the entire wall, and convert that into cubic feet of stone. The multiply that by the density of the stone, and you have your weight needed.
Of everything that's going on there, the liner and it being spring-fed seem odd, and the sandy soils with a partial liner have me scratching my head a bit.
The pic linked below is us strapping a 5,000 pound stone for a waterfall.
