There are two versions of SketchUp
The free version is great for semi professional work.
The Pro Version is a bit more costly (Around $450) and is very full featured design program. Its 3D intersection capability for combining externally designed parts is fantastic. The ability to extrude is enhanced in the pro version. But if you are looking for 3D design tools, I think you should at least look at SketchUp 8.0. And read up on the Pro version. I have used both and I find them imprtessive.
It is not CAD or CNC oriented, but I think you can get bolt-ons. You would have to check.