If money is not an issue, go with HDPlex.

It depends on which Nehemiah CPU, the fastest 1.4GHz variant has a 21W TDP, but the link to the EPIA-M says it only supports up to Samuel 2? That appears to max out at 800MHz and 13W TDP. The CPU VRM likely draws its current from +5V so that's 2.6A or 4.2A (+ VRM inefficiency).
In any case, I've posted before that I used to run a Jetway J7F2 (CN700/VT8273R+) with a 25W TDP VIA C7-D 1.5GHz and a 5400RPM 3.5" HDD on a 80W PicoPSU (the older 1.0b with 5A on 3.3V and 5V, and 3A on 12V) with external 60W 12V PSU. It did need a small fan on the PicoPSU otherwise the hard drive would occasionally post errors. Given that 5V x 5A = 25W I guess the CPU doesn't consume all of its TDP. The system ran for over 7 years as an always-on server (CPU was occasionally maxed out) before it was retired and the PSU re-used on a newer server (which did have an ATX12V input so the fan was dropped).
In your case, you have more demands on the +5V rail:

• 2.5" HDD, probably max 1A
• sound card, probably quite small
• Voodoo (I guess you meant 3 2000). Searching around suggests it can consume up to 15W so 3A
• Slim CD-ROM drive
• Slim floppy drive

No idea how much the slim storage devices consume, they do come in USB-powered variants for perhaps 0.5A each?
The picoPSU 120-WI needs its rails de-rated by 20-40% (so 3.6-4.8A) if run fanless but even 6A is not going to be insufficient. The DC-DC 160W also needs to be de-rated by 25% if run fanless, I suspect you'll need a fan if you use this, especially with the Voodoo.

Thank you for the detailed info!

The CPU is a VIA C3 Nehemiah 1 Ghz (15W TDP).
And you´re right about the voodoo, I meant Voodoo 3 2000.

So... if I´m not mistaken:
CPU: 3A + VRM inefficiency= 4,5A
HDD: 1A
Voodoo: 3A
Slim CD + Floppy: 1A
Sound card: 0,5A?

That's already 10A of power draw in the 5V rail. Even the HDPlex is in the limit.

I´ve been thinking in hard modding the case to fit a FlexATX PSU between the MOBO and the power/reset buttons, giving up the front USB/audio panel. I´ve found a very cheap Seasonic SS-250SU in my zone (FlexATX with 17A 5V rail) so I´m giving it a try.

ayuwea wrote on 2024-08-16, 12:57:

CPU: 3A + VRM inefficiency= 4,5A

The CPU VRM efficiency isn't that bad, probably similar to the efficiency of the DC-DC converters in a picoPSU so >85%. However, some of the power draw of the CPU will be on the +3.3V I/O as well. Taking both into account, it's probably safe to just use 3A for +5V. So that totals 8.5A, still just outside the 8A limit of the DC-DC 160W PSU but it can probably take a brief peak beyond that and you're probably not going to load everything at the same time.
The pico-style PSUs are really designed for modern systems where the CPU and GPU draw from +12V.

ayuwea wrote on 2024-08-16, 12:57:

I´ve been thinking in hard modding the case to fit a FlexATX PSU between the MOBO and the power/reset buttons, giving up the front USB/audio panel. I´ve found a very cheap Seasonic SS-250SU in my zone (FlexATX with 17A 5V rail) so I´m giving it a try.

That would be preferred, happy modding.