Yeah, I had a quick look at the specs and the DEC840 reminds of a standard PC equipped with AMD so it should probably do alright. Maybe somewhat expensive but I guess it depends on the packaging that includes one year of OpenSense Business Edition.
Anyhow, RouterOS CHR will only be able to run as virtual machine thus you first need to install for example ESXi (check if not already installed) or Hyper-V Server (bare metal). If neither works you may install Linux or a regular Windows Server with Hyper-V role, VMware Player (alt. Workstation), VirtualBox, KVM, and others.
Check out the help page for "
Cloud Hosted Router (CHR)"
Ps.. Just curious, why do you want to swap to RouterOS instead of OPNsense?