I once had the help from WarriorBen, it took weeks to do it. I paid him a quarter million and for all the resources needed.
Just as an indicator for others who want to hire people.
a chunk is 16x16x16 to get the blocks in the cube.
The area is 5x5 chunks, so 25 x blocks in a chunk
to make the tank you need 27+ levels mined, so 27x previous answer
which should be around 2 million blocks to fill, drain, sponge, gravel, sand, excavate, place, build and manage.
total blocks / durability of items = quite a few tools needing repair as well. maybe make it clear you provide the picks and shovels, the sand, etc?