Cad Tools and BPT Pro are awesome addons for Illustrator that you may want to look in to.
I have used AutoCad and 3d Studio Max pretty extensively in the past and these addons certainly can't do what full blown dedicated software can do, but the learning curve on a cad or 3d software is pretty massive, where as those plugins for illustrator you can install and start using it immediately without having to learn much of anything really.
http://www.baby-universe.co.jp/en/plug-in/products/bpt-pro/
Oh Mike is correct also, if you want to go the 3d route, Sketchup is awesome and semi easy to learn ... and I believe Google offers the full working intro version for free if I remember properly.