I don't see that case doing much for either interference or immunity either.....
Many plastics do not do well in mineral oils, so making your cables up out of teflon insulated might be a good idea (Also, watch the fishtank plastic, it may eventually deplasticise and become brittle, real mess potential there).
CPU heatink fins run the wrong way for convective cooling (And pull the cpu fan, it will not make an effective oil pump and just blocks the oil flow).
If you must use silicone gunk, make sure you get the electronics grade stuff that does not emit acids as it cures.
I would be really nervous about the PC DIY crowd pulling the guts out of a PC power supply like that, those things do not take prisoners, and the case is actually quite important to both safety and RFI screening.
Also, oil immersed electronics is a messy pain in the arse (The mucking stuff gets everywhere), been there commercially, damned if I am going there on a hobby job, I would almost rather deal with DI water.
Also, remember that you still have to get the heat out of the oil eventually....
73 Dan.