Sketchup looks pretty good and does have a library of buildings around but you are likely limited to "private use" unless you spent time researching any copyright issues. I did a while back and couldn't find anything. So the stuff is copyright, by default. I couldn't see any global site rules about it.
That said, I will just go and do an unashamed bump! of a thread I started a while back with a progress update as my excuse. It's essentially a begging thread anyway!