If you end up using that pico, you will be fine. If you use a mini-itx, the M3 will be a bit on the low side (generally speaking).
Have you checked out the Commell 3.5" boards? Maybe too expensive for what you are trying to do, but nice none-the-less. I have the LS-372 and I use the DSX12VD PSU that has just the P4 connector with no big ATX 20pin.