Just a few things to try before putting more money into the project.
(I really like that case BTW. Very nice.)
As I recall old Seventeam PSUs were pretty reliable.
If the PSU DC voltages are good I wouldn't make replacing that a priority. It is possible for the voltages to be good but the PSU bad due to excessive ripple (bad caps) but as the system is working via HDD I don't suspect that. You could do a visual inspection of the caps. (Make sure it's unplugged from the wall.) Bad caps don't always bloat but bloated caps are always bad.
Sometimes RAM (or add-in cards for that matter) that has been in slots for long periods of time loose good contact due to corrosion. (Even gold corrodes in air, just every slowly.) Usually simply removing and reinstalling the RAM (or card) a few times will correct that by rubbing the oxides off the contact areas. If not the popular solution is to clean the fingers (contacts) on the card or modules with an eraser. Clean the eraser leavings off with a swab saturated with 90% or better isopropyl.
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GRUMPY OLD FART - On Hiatus, sort'a
Mann-Made Global Warming. - We should be more concerned about the Intellectual Climate.
You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.