Can I buy it outright, or do I have to subscribe?
Both. They differ in what happens after launch, not in what you get at launch.
Buying it once means you own the version we build. It is deployed on your server and the code is handed over. There is no future development included — what ships at launch is what you have.
Subscribing means you keep everything built from that point on, as it is built.
That is the whole distinction: today’s build, or everything from here. Current figures for both are on housingmarketdigital.com, and the scoping call is where a quote for your job comes from.