So what are your requirements?
If you just want to run one backup per day and the backup window doesn't play a role, then your current NAS could be enough. But don't forget: It has just one HDD. There is no RAID for redundancy.
If you want to have a smaller backup window, data reduncancy and better performance, then you should invest in a NAS with 2 - 4 HDDs (you just have the two VMs?). That the lowest entry point in my opinion.
To plan the needed capacities you should check http://rps.dewin.me/