Dude! I bought an HP- E-pc for the same reason - until i realised it was a slow piece of junk when i got it running!
If you have the cash to spend on the asus - looks like a good deal..
All you need to do is get on Ebay and get yourself a universal laptop car adapter - preferably a decent targus or something - although the $15 Australian DOllar item i brought ran the EPC for days with no dramas.
19V is generally a laptop power brick voltage - which is why u would use the laptop adapter.
If your keen on a project, there are plans all over the net to build your own, or buy, a startup / shutdown controller.
This is probably an easier way out than mini-itx / M2 - youve already got a case sorted out, power sorted out on the cheap$$ amd a full warranty on the actual PC itself - not to mentioned its engineered for all components to work well together.
Hope this helps. Wish id seen something like it earlier!