A normal AT PSU might not be enough, usually they have 2 connectors called P8 and P9, while here it needs one with 3 (so P8 P9 and P10).
I can't tell you if connecting only the P8 and P9 functions.
Perhaps with an ATX PSU and an AT adapter it could be the solution, but you should find one with the P10, or else modify one and add it (knowing which wires to use).
Looking at the motherboard I think it's an IBM Intellistation, maybe there are 2 similar types, yours could be a Z Pro or 6899.
You should also have the Riser card to install a PCI Video Card, I don't see integrated VGAs by eye.

Here is the manual:
http://ps-2.kev009.com/pccbbs/commercial_desktop/6899tim.pdf

It took me 1.5 years of struggle to start this board.
One year i spent to find of original IBM PC 365 PSU. But no luck, few POST codes after power on and then silence.
I tried few dozens of different memory modules with no success. But few days ago one "gold scrap" fan offered me few interesting memory modules. I gave it a shot and god damn it works!

So there are 2 conclusions to be made:
1. This board requires two extra 3V power supply. So only oroginal PSU will do the job. It does not work with standard AT PSUs.
2. It does not works with any SDRAM. Looks like it requires rare SDRAM with EDO chips.

SDR-Sdram is SDR-SDram. Usually found in DIMM-layout. DIMM or SIMM is just the type of form factor. It has nothing to do with the type of memory on the PCB, just the physical characteristics.

What you need is EDO-Ram in dimm-layout. It used to be quite common on Ebay. Just make sure you know which voltage and speed (50 or 60ns) to look for.

Hi,

Can you elaborate what is the difference between the original PC365 / intellistation PSU compare to the AT PSU?
I have the AT PSU with similar power connector as attached (P8, P9, P10) and 5vsb + PSON 3 pin connector, I wonder how can I connect the standby power + pson to the board.

without opening the power supply, the 365's psu is a proto-ATX unit with AT connectors.

Acer had some weird crap like that in their circa 98 prebuilts. Though that's only by seeing it looking the same, no idea if it was wired identically.

I am looking at the manual, it seems that there's a 5V auxiliary power next to the power switch connector on the motherboard (close to the PS2 ports).
Is this the one that supplies the 5V standby and ps-on power as in the ATX power supply?

Yes same thing way just like ATX is.