Tips for building
I have been off the forums for awhile, life to do and such... sounds like you are all-in on this project! Post lots of pics of what you do.
The heat exchanger in the water tank will do its best at the bottom. When cold water enters, it naturally sinks, so that's your best spot for heat transfer efficiency. Many DIY people have left the heating elements alone, by either inserting the hx loop through an existing fitting (inlet, outlet, or drain), or by bringing the water to the hx and back with pipe and pipe fittings. The big purpose is to have "less custom" fittings that don't take machining tools to fabricate. You can then buy a prefab, pretested hx that will be a proven winner. Less guesswork for the wary.
With a hermetic compressor and just a single refrigerant path, oil control is a non-issue. The smallish compressors have a copious sump built into the shell. The built-in muffler also acts as a suction line accumulator. With the evaporator dropping way down into the ground, I would size the tubing small to maximize velocity to keep the oil from pooling in the bottom of the hole. A curly-q every ten feet of height would trap the stuff stuck to the walls. Might not need it, but it's cheap insurance.
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