Sounds like you need something like SolidWorks or Inventor. Blender is a cool program but it is a subdivision program like 3DMax and Maya. Not really the same as a part modeler with assemblies. SketchUp does not have these either but it does have a very robust component system. Now neither of these are parametric like SW or Inv. There are a few free programs like this out there. One I recently looked at was FreeCAD which might be worth a look. Would Inventor Fusion (comes with AutoCAD 2012) work for you? Perhaps a few other with more experience, like JD can chime in and give some other ideas.