I would recommend avoiding XAML for 3D models if you can. In its typical Xml form, Xaml benefits from being human readable but suffers from slow load times. This is generally true for all plain text formats that require parsing and reflection. I suggest you look for libraries that can convert common 3D formats into WPF3D format at runtime. Alternatively, you could use Baml or a custom binary Xaml format. The latter is possible in .NET 4 due to the refined Xaml object model and extensible reader/writer facilities.
Loading and rendering 3D models in WPF should be relatively painless. Textures, on the other hand, may be more problematic. WPF3D's support for 3D surfaces/textures is relatively simple, and you may end up having to rework a lot of textures if you rely on free 3D assets from the Internet.