I could potentially get a single SSD but installed directly in an ESXi host & configure a VMDK on that
Actually, SSD is no longer a requirement.
I know WAN-X on VMs isn't recommended
May I ask you why do you think so?
As it's stated in helpcenter
To create a WAN accelerator, you need to assign the WAN accelerator role to a specific machine. You can use any 64-bit Windows-based machine in your environment, either physical or virtual. You can even assign the WAN accelerator role to the existing backup proxies and backup repositories
So I think your VM-based WAN-X will be fine with non-SSD disk, however you can try to use SSD if you want.