Thank you for using GroupDocs APIs.
Groupdocs has launched new Generation APIs starting from version (3.x.x) which are totally changed and Independent of User Interface as legacy APIs (v2.x.x) were UI based APIs. Most of the features are already available in new GroupDocs APIs and some are being incorporated in every new version with many other new features.
Currently comments rendering feature is not available in GroupDocs.Viewer for .NET but it will be available in upcoming release of GroupDocs.Viewer for .NET for words document format only. If you want to render different types of Annotation with many other document formats, yes then you will need to have a GroupDocs.Annotation for .NET API license and we also recommend you to go with GroupDocs.Annotation for .NET as it also renders document without need of GroupDocs.Viewer for .NET or you can chose GroupDocs.Total APIs which will provide you not only Annotation and viewer feature, it also gives you many other APIs in single license.
Regarding licensing and purchase question kindly create new thread on this forum.
Have a nice weekend ahead.