The Solo can fit both standard and Micro boards?
Is there any advantage to getting the non-M version of the board?
Usually an ATX board should give somewhat better reliability, expandability and mechanical compatibility/workability.
IIRC the ATX have better power regulation, USB 3.0 on board, instead of the external PCI-E card as the M does, and it is CrossFireX certified. It obviously have more expansions slots (mainly for a better hw placement).
And is there any great advantage to the ASUS M4A89GTD PRO/USB3?
Over the 880M? I think it has a better IGP, 1 SATA port more, a better chipset cooling (heatpipes), slightly better overclocking capabilities, plus the not-M version above quoted.
Just as a side note, ASUS site have a comprehensive product comparison feature, you may use it.
The lowest wattage Enermax I can find over here is the Enermax MODU87+ 600W, which is pricey and way way more power than I'll need, but then I think they come with a five-year warranty.
They are not on the same league.
The Modu87+ you've found is a modular high end unit like the Seasonic X ones.
The ECO80 I advised for is a somewhat more basic unit with a very favourable noise level.
In the between there's plenty of choice (even if mostly Seasonic based, I guess).