Hardware is always a nice solution, until you need to migrate, upgrade or expand. Using VMs can be tricky, but any critical system (monitoring, database, etc) on virtualization should be able to function without issues if there are enough resources available. The comment made by @martin.schwarz is the most sensible one. There is no point in “learning” a new OS, just for your monitoring system.
Yes it is linux, but the differences between Debian and Suse or any rpm-based distro are numerous. That said… SLES 11 is nowhere near recent, even with the Service Packs.
So make sure there are enough resources for your CheckMK VM, use a recent distro you are comfortable with, that also fits within your company strategy. (Or go with the CheckMK virtual appliances)