I see we have a recommendation for minimum S2 SQL Azure when hosting on azure. But can't see anything regards minimum app service plan? Is that simply because we can run on the free tier upwards and soley dependent on the enduser load, with umbraco core having no particularly specific minimum memory/cpu requirement?

"It depends". I've started with an S1 App Service Plan with a B (5 DTU) Azure SQL database years ago with Umbraco 7. The backoffice was a bit slow but for a small site with not much traffic it would be good enough. These days I would recommend to start with a P0V3 server. The Premium V3 servers use the latest hardware and offer the most memory and CPU for the lowest price compared to the older S, P or PV2 series. Combine the PV3 with Linux as the OS, a reserved instance for up to 3 years and you can get it for as cheap as ~35 € / month. Check it out on the Azure calculator: https://azure.microsoft.com/en-us/pricing/calculator/