Apparently, RouterOS will support all that.
I wouldn't mess around with a miniITX. What for? Now you have to concern yourself with hardware, drivers, etc.
Grab the pre-fab unit direct from Mikrotik. You can always phone them first and confirm that it will provide you with all of the necessary feature and functionality you're searching for.
If you're set on going the mini-ITX route, why even pay for RouterOS? Just install a free Linux-distro and build a multi-link PPP router yourself... ....or install PFSense (also free), etc. There are even other options as well, such as ZeroShell (Web Interface, so it's easier than using command-line to create necessary rules yourself), Slax Router (another Linux run-off --> »slaxrouter.sourceforge.net/galle ··· ery.html
There are more options, but you get the idea. In all honesty, I'm looking for an MLPPP bridge device. I'm waiting for the day when someone (Thompson?) builds a dual- or quad-port DSL modem, with an internal MLPPP bridge configuration, so that it simply provides the MLPPP link, and forwards all other traffic (including DHCP / WAN IP) directly to the firewall sitting behind it.