The 2012 version of AutoCAD does not have a mesh fixing tool. Was never truly intended for the 3D printing world. That could change in future editions.
Mesh Mixer is an Autodesk product that can do some mesh repair ... not too good at fixing Sketch Up files.

What will help your work flow is a plug in I just saw in another thread
http://www.susolid.com/

Blender does cutting and boolean modifications
the user interface is very different and takes a bit of practice

NetFabb is quick to learn and you don't have to wade thru a bunch of UN-needed functions
there is a free version ... that won't do what your asking to do ... but it's worth a look