Hi Alex,
as Christoph already said go with Flexible Server, Single Server was deprecated some time ago.
you don't need to worry about availability zones, AI or replication if you don't want to use it. Just create a single instance in the region of choice (availability zone will be automatically assigned) and try it out - there is a free tier for a year to try it out.
https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/how-to-deploy-on-azure-free-account maybe you can add it to your PoC
there are no 3 options: "Azure Database for PostgreSQL" is just an umbrella name, not a product, the product is "Azure Database for PostgreSQL - Flexible Server". Basically it's just one for now.
regards,
Alicja
