You could also look an AMD E-350 board like the Asus E35M1-I. It has 6 sata and is fanless. Just be aware that it is MUCH slower than even a Celeron. It's more than enough for file and print sharing, but if you might want to do more later it might not be fast enough.
There's a low-power version of Zacate called Ontario. Asus has a fairly cheap board based on it which should be good for a NAS.
Other fanless Asus boards are pretty expensive considering the cooler you get. Asus isn't very good at this but I guess their Ontario board should be OK since the lower heat output should in principle be lower.
ASUS C60M1-I might be indeed fairly viable alternative, although I would have liked to have HDMI output...
In 400+ W class, there would be even these fanless options, but how well the fanless option would work in this small case? Not to mention that perhaps I would prefer something with slightly less expensive...
Power supply availability can be an issue in some locales but there are fanned PSUs which will operate witout spinning their fan for your power draw. They're pretty expensive already but the fully fanless 400+W models are ridiculously overpriced.
I'm not really ruling out any PSU options. Compact size and modular cabling are the two features, which I am mostly interested in since the small case doesn't have too much extra space. And somehow I am reluctant to invest in any high wattage (say more than 550W) unit when the power demands are clearly much less.
I don't think you necessarily have a use for a noiseless PSU anyway. Your drives are going to make some noise. Can't you put your NAS somewhere where the noise won't bother you? That's kind of the point. For that matter, I don't think you have a use for an Asus board if you go 1155. I'd either money on an efficient brand to save power or pick a cheaper one.
All the other 1155 ITX boards I have looked at seem to offer less SATA ports than that ASUS model and if I would select such board I would need to add a RAID card and I'm not that interested in that option, although most likely then I could utilize hardware-based RAID rather than software-based.