Your cab is fixable. However how much time do you want to put into it? My philosophy is to stay away from the rust buckets and start with something that is relatively rust free. I would rather put my time into fixing the mechanicals. A cab in your area that was well undercoated will be surprisingly free of rust. I am hesitant to replace floors with the cab off the frame. It is very easy to wind up with a cab that won't fit the frame or the doors won't fit. One of my projects is a 50 GMC. There is no rust from the bottom. However moisture stayed under the floor mats and rusted from the top. I am leaving the cab on the frame, replacing one side at a time to keep everything square. I have seen cabs so bad that there is nothing to weld to, and yours could be one of them. As far as bringing one in, watch this site.
http://www.ytmag.com/cgi-bin/haulquery.pl You can usually get things moved reasonably. You might consider buying a whole parts pickup. There is a lot of little things on them if you have space to keep one.
