Hi Ambi,
I am also interested in a Sketchup-only workflow. I think it's possible if you are careful about how you model in Sketchup. If you model from scratch with the idea of making a "manifold" model, it should be a smoother workflow than trying to turn an already completed model into a manifold model.
I don't think external tools are a "must"; but you need to have a good understanding of Sketchup, and what it means for a model to be manifold. The way Sketchup works can work against your desire to create manifold geometry.
For example, Sketchup will create a face (or faces) automatically when you close a shape. Sometimes, this face is internal to your model and there is no indication the face is added; but internal faces make the geometry non-manifold. You need to find and remove internal faces. X-Ray mode can be useful to do this; or use a Section and Move it through the model to find them.
There is a great plugin for SketchUp that can help in cleaning-up a model of loose and un-used geometry. It is called
cleanup. It's a must-have plugin as far as I am concerned.
I am not sure how Shapeways handles Groups/Components, I'm still experimenting with that; as well as with which collada export options are optimal for Shapeways. For now, I explode all Groups/Components before exporting.
I don't know if Shapeways' collada importer supports image textures. I know if your model uses only colors in Sketchup, the colors are preserved and shown in Shapeways' preview image - so I assume they can be printed using the full-color material.
If image textures are used, i think you would need to edit the .dae file and the location of the texture files, and create a .zip file of them all to upload. But I haven't tried this yet.
Hope this helps some.