NAS: Off the shelf or homebuild?

Hi folks,
I've been looking at nas options for a little while now. I'd be budgeting up to €200 (excluding HDs), so most of the large capacity prebuilt options are out. I've been looking at an icybox nas enclosure at the moment (link). It can do everything I want right now, but I would like something with more expandability and flexability if it's possible within the budget. The other priority is power efficiency. I've set myself an absolute limit of 25 watts idle draw, which rules out re purposing an old Pentium 3 or some such.

Some have suggested mini-itx/micro-atx boards with built-in or mobile processors (example), and I really like the idea, but i'm really struggling to come up with a decent looking SFF case+PSU that will and fit 4-5 HDs and the budget.

So, should I stick with the prebuilt, or can somebody have better ideas?
Cheers