Not sure if I still fully understand you, but as it relates to creating C# Web APIs and non-visual C# assemblies I think everything PB provides is sufficient and I don't see a strong need to purchase VS C# IDE unless you need other project types besides what I mentioned. 

Other project types would be for example ASP.NET, UWP, WPF, etc.  Just FYI,  the 4 key project types that will be in the first release of PB 2019 will be C# Web APIs, non-visual C# assemblies, C# unit testing, OAuth2Server.  In the revision (second release) of PB 2019 we plan to add ASP.NET development capabilities.

In case you need ASP.NET development capability and can't wait for PB 2019 R2, one option to save money is to use PB 2019 for all the heavy lifting and then use the free Visual Studio Code for the ASP.NET page development.  Although you need to work with two different IDEs, you will still get the key productivity advantage of PB because the advantage of PB is on the business logic development not UI development.  Also, this would just be temporary until PB 2019 R2 hits the market.

The C# feature is for creating non-visual assemblies and REST Web APIs.  The two features it provides is full-fledged.  Specifically, you can use any C# feature you like in addition to what PowerBuilder's C# framework provides, and the C# projects you create can be designed to be cross-IDE (e.g. you start developing in PowerBuilder IDE and then your project maintained in Visual Studio IDE).