No. You'll only need it if you chose to upgrade to one of paid plans.
That is because you can test all the features of the plan by using our development images that we provide for local development. The images a limited to only one database connection.
It means that you pay a one time fee for the subzero stack and you can deploy wherever you chose and scale as big as you need. The only condition is that it's used for a single production deployment/project/database/api.
Yes. You can use the stack indefinitely, the only limitation is you get version upgrades and commercial support in the first year only.
No. On Premise plan means you can deploy it only in one production location (staging does not count). Your clients will need to purchase a new license. If your service is a type of SaaS business, that counts as a single location, even though you might run multiple container instances for scaling.