In this particular context we have a small business (10 user office) looking to establish a Domain setup. There is already one machine running Server Foundation 2008. If that machine is selected as Domain controller can we purchase another running Foundation 2012 to serve as a backup or would it also have to be Foundation 2008?
You can mix windows versions all you like in AD domains. The only constraint is that the functional level of the domain and forest may be at most that corresponding to the oldest windows version used.
So, yes, of course you can do that.