Hi there, nice system.

Before you go opening up your PSU, can you take a photo of your motherboard and show us the capacitors close to the CPU? They might be going bad.

b) As far as the monitor goes, it seems like it has seen a lot of work. You can usually confirm that by going into the service menu, if one is available, and seeing the runtime/hours. Likely, this one has been used quite extensively and as such has deteriorated over the years. Not sure if there is a lot that can be done, you might be better off sourcing another one in the event that this one decides to hang the gloves.

Sure thing:

Let me know if you need other pics

As for the monitor, I'm trying to get to the service menu but so far couldn't find the service manual for a 750s

Edit: seems there is no service OSD on Samsung syncmasters and everything is done with Softjig.exe, a signal generator and an interface board that's way out of my reach

Edit2: Checked for bulging/leaky caps, apart from a bit of dust they all look flawless. If it's the power supply I won't have the technical skills to repair it so I'll have to use a new one

I am using a thermaltake 430W PSU from round 2004 in my SS7 build, and it does the exact same thing. The power button is unresponsive for a few seconds after turning on the power supply. I'm not sure if it's the mainboard or the power supply but it never seemed to cause any other issues so I just live with it.

I suppose to test if it's the mainboard or PSU, you can check if the PSU turns on instantly by grounding the green power on wire and then switching on the main switch at the back. If it comes on instantly it means there's a delay with the mainboard being able to switch on the power supply.

I have a tested enermax 300w on the way, I'll swap them and see if it fixes it... as you said apart from the 10s wait time the system works fine, but still I'd like to know if it's normal behaviour.

Dleatherhoff wrote on Today, 09:32:

... When I turn on the power strip after a period of inactivity, pressing the case power on switch does nothing, I have to wait 10 seconds and then press it for the system to boot normally.
If I shut down the system and power it back on shortly, it boots right away. It really looks like the PSU needs 10s to "charge up" and then it's good until I shut down power to it which causes it to progressively go back to "uncharged" state.
Could anyone shed some light on this behavior? Capacitor issues in the PSU? ...

Yep, PSU caps are bad and their ESR (resistance) is too high to start.

Once they're warmed up (like when powering back on right after shutting down), ESR decreases enough to allow starting.

A trick as old as the hills:

Blowing a hair dryer into the PSU

Your PSU needs to be recapped or replaced as soon as possible.

Thanks for the info, enermax here we come! From my research that's a good brand and the seller tested it before shipping.