I want a MorphOS exclusive machinem G5 PPC is a perfect for that. I just want to know what type of G5 PPC is 100% compatible and what is the best configuration, optimal to maximum when it comes to GPU, etc.

Thanks in advance. Oh, I don't want water coolent because of the issues with them, what is the best CPU fan coolant for this machine?

You need to decide what you want. 100% compatible or "optimal to maximum when it comes to GPU".

AGP G5 are better supported, but GPU beyond R300 (Radeon 9600) are harder to find as genuine Apple cards (which you would want). They end with R500 (Radeon X1___).
You will also see higher power consumption at the same clock.

PCIe G5 use less power, and on GPUs they go as far as driver support allows.
You need a PCIe NIC (7€ new) and a supported Radeon (10-30€ used). Keep the NVidia in so you can get a bootmenu when needed.

If you get a 20-inch iSight iMac G5, OP won't have to worry about anything, really, to my knowledge concerning MorphOS support. But, many times an all-in-one computer isn't the most convenient nor desirable alternative, since we may have plenty of monitors at our disposal that merely need a standard sized computer to plug into. So, a single-core 1.8 Ghz PowerMac G5 it is, then?

I just want to buy that fat, heavy tower PPC G5, 7.3 please. If anyone have it for sale please tell me. I am interested. I will pay for shipping cost. I only want MorphOS machine anyways, just MorphOS so this hardware is more than enough for me.