I actually have different answers, based on the case that we would get at work. I'll try to get you some info on it Monday, IDK if our computer dealer will give me any.
1. We buy full systems from a dealer in North Dakota, they still build new machines in the case so they either have a ton of stock, or are still able to order it.
2. The PSUs in the case are a standard server form factor. Rather than replacing it with a HDPLEX style unit, I'd look into sourcing a 1U (I think it's called?) PSU that has your target wattage, so long as you're doing AC-DC power conversion. Might be an easier process overall, and you might be able to swap a noisy fan yourself. That's a matter of preference, though.
3. The cases we order come with two "+" shaped pieces attached, one on the backplate and one on the "bottom", they both can be removed with four screws and they can attach to every surface but the "top" and "front." They fit into a Vesa (100mm and 75mm) compatible "X" and rotate to lock into it. We use these mounting fixtures at my work and they stand up to probably a lot more abuse than I would assume you would put it through.