I work in a medium size historical society and I often use a very cheap
CAD program to "sketch" ideas.  Its by a group called EXPERT software - I
found it in the cheap section of a national software chain in a mall.  It
cost about 20 bucks and is not real fast.
I don't use it as a drafting program but as a sketching program-after I do
a virtual walk through I modify the design and work in a drafting program.
I have not seen any high end CAD prgorams myself.  I always wanted to be
able to scan a picture and put it in a virtual exhibit!!!  Just a bit
beyond our museums budget (or needs.)

Good Luck
Patrick Reynolds