Hi Andy,
OS Property is Joomla extension, so it doesn't come up with any Joomla template. In case you purchase OSP Templates (OSP Home Search, OSP Real Homes, OSP Real Estate ..etc), you will have both Joomla template and OS Property extension.
So, in general, you just need to purchase OS Property extension when you already have suitable Joomla template while you can purchase OSP template when you need a real estate site (with full real estate features with suitable template)
Thanks
Dam