Is this being hosted in a VM that would be on-premises or cloud hosted? Is this going to be Windows-based or Linux/BSD based? It generally does not take a whole lot of computational horsepower to handle a lot of DNS requests. If you’re doing to handle DNS using Linux and don’t need dynamic update capability, you would be just fine using something like Unbound or NSD. I’d recommend maybe 2G of RAM and minimum 2 cores allocated. Now Windows will be much different. You’ll need more horsepower than that.