Hardware should be tuned and adjusted depending on how many VPSes will be (or potentially can) be hosted on the KVM.
SolusVM Slave Software itself is rather modest in terms of the resource usage, it can be neglected.
Therefore, refer to the Official KVM requirements for the details: CHAPTER 1. SYSTEM REQUIREMENTS
You should consider checking Partitioning for KVM Slave part as well before purchasing the server.