You forgot about hard drive, and ram, and all the other cable needed.
I really don't think you want to put your OPUS in the enclosure. You could put the ATX connector on the outdside of the case. That was it would still be removable, and you could hardwire the radio connections to the OPUS. If you do end up needing the tank circuit, you could always mount that some where else.
You might be able to get the dongles in there, but you are gonna want to have bluetooth and wifi mounted remotely though. Or at least the antennas.