The LAN bonding is called link aggregation, and yes, the 2413+ has that, although you need a switch with a IEEE 802.3ad standard, and as I looked around a bit, they're quite expensive. On the Synology site, they mention 3 of these switches which are compatible with the link aggregation function, the 3com 4200G, the HP Procurve Switch 2824 and the Voltek NSH-2926.

The rack mount NASs are also very expensive, that wouldn't be the best solution.

I planned on selling my QNAP, as I wouldn't keep it as backup, because I wanna run the 2413+ in Raid6 mode, so that we be already quite secure.