Maybe that will more easy with Sketchup who has a system of components who reduce the size of the file of the buildings !
It's free and specialized in architecture!
http://www.sketchup.com/
Moi has not that replicated instance components yet! ;)

About your "big" DXF file ( 53 megas! ) you can reload directly it inside SKetchup but on my old (10 years) computer I have no result after a reasonnable time (30 minutes) !
Try it on your modern computer!