Some good news to report: After connecting a spare PSU from my inventory [OCZ 520 - peak supply: 620W] everything is working, including the DVD-ROM drive that previously wouldn't power up. It "simply" remains for me to remove the U/S unit and replace it with the functional one. My current case has the PSU compartment under the motherboard compartment, in its own bay with dedicated air flow. It's a neat case, huge but a heavy bastard. It came with its own PSU, which due to the configuration has longer cables. As the OCZ unit may not have cables long enough to reach to the top of the case I may have to swap the entire contents to my other case. Either way, I've got a bit of work ahead of me this afternoon. But that's small potatoes compared to my original horror: having to replace a motherboard and/or other expensive components.
Thank you to all who offered me advice.
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