Hi
Being a "Blender head", I understand the learning curve problem from personal experience. I recently found an effective solution to the hollowing problem, though: duplicate; never extrude. If you select the faces, duplicate them (SHIFT+D, 0, ENTER), then ALT+S to scale every face
along its normal, you will usually end up with a close-to-perfect shrunken object. ALT+S even measures by units so instead of trying to get wallthickness by percentage you can just type in the mm representation.
You'll need to fix any faces that have scaled
through other faces, I just merge the points together on the inside object so it doesn't get funky. Another possible problem is the inside/outside calculation for faces in Blender - sometimes unreliable. If your model is watertight before duplicating and comes out right already as an STL, make sure you flip normals of the new faces (W, 0).
The last part of this of course is connecting the inside and outside through a hole. Cut out the faces on both inside and outside in one spot, then connect the vertices (select 3-4 verts and press F).
I'd be happy to write the Blender tuts if no one else gets to it before I have time - I know I'm not the only proficient one.