I am a newbie to creating power portals and have a few questions about the same -
1. Currently, I have PowerApps for the Office365 plan. Is this sufficient to create a new portal (with a database)? If not, what other license would I need?
2. What should be the ideal portal lifecycle? Create a portal in the sandbox environment (during development) and then move this portal to the production environment (once it's ready to be shared with users)?
3. Is it sufficient if I buy the Power Apps Per User or Per-App plan when I am ready to share the portal with other users?
no you need a separate license per portal. There are two types you can buy and both have consumption based limits. It is either anonymous accesses or authenticated accesses. Look here and scroll down to portals.
So is my below understanding correct? For the development of the portal (with database), I need either Per-App or Per-User plan along with my 'PowerApps for O365' license. And in the future, after the portal has been deployed for external (or internal) user access, I would need the capacity based plan (Login or Page view based)