One thing to keep in mind for CPU consideration is that it takes one physical CPU to back up one VM disk. I suggest getting a CPU(s) with as many cores as your budget allows. In this case, more is better.
Speed of the CPU is less important although I would say that you'd want a minimum GHz of 2.0. I don't think it's worth the expense of getting a 3GHz or higher CPU, it won't improve backup performance.
As to memory, again, more is better. Count on 2-4GB per VM being backed up.