I'm also not sure about that. The A500 KS1.3 rom will be missing support for the ECS chipset, the IDE HD controller and the PCMCIA slot, so you would instantly lose all of those, plus you would no longer be able to boot ECS/Workbench 2 software. Much better to stick with the built in KS2.05 in the A600 and use a relokick 1.3 disk if you do have any games that require KS1.3 (although many WHDLoad installers fix games so they will run on newer Amigas, so that isn't such a problem.

The only thing on that list I'm not sure about is the A1200 motherboard. Yes it would be the best option, but they are quite wide motherboards so wouldn't fit in most separates/midi or sat receiver sized cases. The A600 motherboard is much better suited for this purpose.

I do like the idea of using an Amiga to PC PSU converter, and that gave me an idea. How about using a mini ITX PC PSU? That way you should be able to fit the PSU inside the case with the Amiga! If that could be made to work it would be a very neat and tidy solution. In fact you could even use one of the many available mini ITX cases for the project instead of converting a case form something else. That way you might even get one with a floppy drive slow already in place (and even a CD/DVD slot).