I know there is a best practice that discourages an environment with only one domain controller.
Why? Do you really need two domain controllers? How many authentications are you doing? How much downtime can you afford? Would it be better to have a single domain controller on a VM that you can backup and restore in a few minutes versus having two running at all times?
Why = because a document from Microsoft said so and at the time when I made our domain I didn't know any better .
What you're asking me is what I'm asking myself, which moves me to the conclusion that when it's time to make the VM for the accounting software, the old box should just go away. Especially since my tiny number of users would be able to log into their workstations with cached credentials until I can get the domain controller VM functioning again.
Who cares what some paper from the company selling you the licensing says.
What does your company need?
I have never used two domain controllers in the SMB space. Even before virtualization at my clients.
It is simply not something needed.
You don't think the downtime justified the cost for a SMB I'm assuming and load balancing isn't a concern
Rarely is downtime worth the cost of mitigating it in an SMB environment. They often don't actually understand what the true cost of downtime is and exaggerate it more often then not. If you're getting enough requests that you're hitting a performance threshold on the domain controller then you may be out of the SMB space.
And authentication often has a near zero impact for short durations. A DC down could easily go 30 minutes and literally have no one notice.