You can skip the Purism laptop, and just simply download the Qubes OS installer and try it out on whatever system you have. It uses the same installer framework as Fedora. As long as your system supports VT-x (pretty much anything recent does), you can have the Linux + Windows experience and the isolation offered by running them in separate virtual machines.
There are more advanced security features, such as isolation of network adapters from the rest of the system, offered with a system that properly supports VT-d (aka IOMMU). Between having a CPU that supports VT-d, BIOS correctly configuring VT-d, and ACPI tables being correct as well, finding such a machine can be a little more challenging than you expect. I suppose that is where some value is offered by the idea of a "Qubes certified" laptop.