DIY PC case for 3 systems?

I just have a few questions about the general idea.

So, I currently have three ATX PC systems, each in its own tower. And since I'm redesigning my room soon anyway, I'm thinking about making a homemade case for all of the PCs.

This is about my main PC, which I use for everything, with multiple PCIe cards, and two other PCs without PCIe cards. Specifically, one PC for hosting game servers and one for cloud storage.

I can work out all the questions about the exact implementation later, but my questions that remain are:

1. What material should I use? Weight doesn't matter since it doesn't need to be moved?

2. Is it worth using one cooling solution that cools all PCs together or is it better for each motherboard to have its own cooling solution?

3. I briefly considered using one power supply for everything, of course taking into account all consumption at each voltage, but the idea sounds a bit complex to me too?

For shared cooling, I had more ideas: a parallel water circuit with three pumps and two 360° radiators and an expansion tank. Or three pumps and three radiators with one tank.

To answer a few questions first: I'm actually quite knowledgeable in this area and am just looking for ideas to refine my project.