Try CaeLinux http://www.caelinux.com/CMS/ a bootable CD distribution which is basically a collection of different open source Cad apps
I'm hoping to make a reprap 3d printer at some point, so I've been looking into writing some ebuild scripts to get some of the stuff from caelinux into gentoo
like Salome, elmerfem, brlcad
I'm not an expert in CAD mind you but some of the below may be useful
One of the things I've discovered is that there's a difference between 3D Cad and 3D Modeling software
Modeling is about approximating the appearance of an object for appearance sake only, usually using a mesh / grid of some kind, this is a typical use for Blender
CAD is about what the object is made of, and it's physical dimensions in real space, typically objects are constructed from primitives such as a hollowed out cylinder for example
Modeling = what you can see the outside appearance, CAD = the innards, what it's made of and more of a focus on measurements in real space
If your going to design something that's going to be built it's probably better to design it in Cad software first
then convert it to a modeling form later on for the sake of pretty pictures / animations / appearances in a demonstration etc.
This way the original design is stored in a form where there's actual physical measurements (in mm for example)
and in a form that can be manufactured (drill holes at these points here and here etc)
While Blender could in theory support CAD capability, I think it's current features are lacking in that area
(although it is open source so if you want to add those features go right ahead)
From a commercial perspective I think the 2 main packages are Catia and Solidworks
Also If you want to simulate the environment on a 3D Cad object, the usual way is via FEM or Finite Element Analysis
This is the sort of thing used to simulate the way temperature travels through an object made of different materials for example
I think Catia / Solidwords have this sort of thing already inbuilt, in the case of open source software there's a lot of separate packages to play around with (elmerfem for example)
I think the linux cae pages have some good tutorials / examples on this
http://www.caelinux.org/wiki/index.php/Doc:CAETutorials