Does starting up a new project by following this article work for you: https://docs.telerik.com/blazor-ui/getting-started/client-blazor? Once you have that running you should be able to add other projects and references that you need.
If it does not - I advise that you uninstall .NET Core 3 RC 1, restart the PC and install it again, the error you get seems pretty generic and there may be some issue with the framework installation.
On the version conflicts - the Telerik.DataSource package depends on Microsoft.CodeAnalysis.CSharp.Workspaces and we reference the latest version. If this raises a conflict when adding it to an existing project, this means that the project relies on an old version and should get updated first. For example, it uses an older EF version.
On choosing a framework - I will assume that you are referring to choosing between Server-side Blazor and Client-side Blazor. What I can say on this is that they are fundamentally different and I personally find the server-side flavor to be a stepping stone so people can try out the concepts earlier. The true innovation and the actual SPA framework is going to be the client-side flavor that runs on WebAssembly. It is, however, not ready for prime-time, the latest news I know of indicate that it may officially ship in the Spring of 2020. So, for the time being, I'd suggest making a proof of concept on the server-side blazor flavor. It will be supported officially in the upcoming .NET Core 3.0 release (that is expected to drop before the end of the week).
If you are referring to third party toolsets such as ours, what I can say is that we are building native components from the ground up so we can follow the emerging best practices. We decided to not wrap jQuery widgets as this will clash with the Blazor framework. We are using the experience we have from our other web toolsets to get this off the ground as fast as possible (for example, the grid has all the core features a grid needs about 6 months after its initial release).
UI for Blazor