I have some really nice ATX power supplies laying about, it would be nice to be able to use one of them, but I am nervous about frying this machine as I have wanted an SE/30 for a long time and finally was able to pick one up for a good price.
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The problem I see in a potential recap of your board is that a fair amount of the bottom copper is rusted/rotted underneath the green solder mask. A recap alone without addressing that problem would not be advisable. The condition would grow worse over time and it also could prevent even a recapped board from working properly. You would need to try to carefully scrape off the blackened solder mask to examine the copper underneath to determine how much of it is left.
As to use of your existing ATX power supplies, assuming they would physically fit, you will need to tell us the electrical specifications so we can tell you if it would be safe to use. Also keep in mind that many ATX PSU's have a fan, and whether that noice would be a bother to you is something you would need to decide.
I now see that the only SEASONIC available on Amazon is the
SSP-300SUG, selling for a rather high $109. I used the SSP-250SUB which cost about $49+shipping. The manufacturer's web page doesn't say anything about the product being End of Life, so I sent them an email just now to inquire further. I told them that their response to me would be presented to the vintage Mac community, and that a price of about $49 is what we are looking for. Let's see what they have to say.