Cool the PSU? You don't want any liquid by the PSU in case it fails(leaks). As for water, most tend to add some anti-fungi killer(anti-freeze) in order to control the growth of such. AND/OR use a specialized liquid that maybe part of the kit or some additive. While you can water cool, having back-up air fans, will do fine just to keep it all together.
I wouldn'ty cool the PSU offhand, but everything else can be. However, I tend to think, BIG PC case in order to mave room and additional areas to place fans and/or mount whatever. Plus, a decent case may lend itself to custom builds thus you start from the ground up rather than thinking GPU and CPU stuff, having a good case and PSU is a must, IMHO. Further, once you start stuffing the case, you rather not move it over to something better later, you'll probably keep the case as it came, so having a better case from the beginning is a good idea.
Of course, you could think "oil immersion" placing entire PC in oil vat. google for examples.