The Singularity and FireStorm viewers hace tools for building in world 3D designs that are plenty when starting to work with 3D design.
Later Blender might be required for converting 3D model formats or detail level, if our render server that automates blender does not meet your needs.
Shared region modules
OpenSimulator is developed in C# and all you need to start extending the platform as an developer is Visual Studio Community Editon, the .NET framework and Git version control. You can then create your own shared region modules that extend OpenSimulator, with any functionality you require.
If you develop your owen extensions will will probably have to host OpenSimulator on your own servers, but cloud services are cheap these days, so it is possible to do most anything.