If you are going to be converting a bridgeport to CNC, why not just purchase one that already is a CNC? It would save considerable time and money to go this route. These are available relatively cheap, at least here in new england. They can be purchased for $2-4000 US, or less if you are lucky. Plus, HSM (Home Shop Machinist Magazine) recently did an article about stripping down and simplyfing a used CNC bridgeport. I am currently doing this to a 1993 model V2XT which is servo driven. The great part about my machine is that the computers were unreliable, so this machine spent most of it's time sitting idle, so the machine is in great mechanical contition. I plan on upgrading the computer control, and I am researching servo drives and controls.
One way to tell the condition of the machine is to carefully look at the ways and the adjustable gibs that tighten the ways. On most of the CNC machines, the ways are hard chromed and all of the hand scraping on the ways is still clearly visible. Plus, CNC machines usually had the automatic oiling system, ensuring that the machinist didn't forget to oil the machine every few hours. Regardless if the machine is manual or CNC, if it has ballscrews or leadscrews, check the backlash of the screws. You can do this by hand, but a magnetic base dial indicator would give you an exact reading. Also, one other thing is to check and see if you can feel rotation of the table. To do this, crank the table to an extreme, unlock all of the locks, and push forward and back on the end of the table. If you notice movement, you can figure which set of ways is loose by the process of elimination using the lock screws.
If you do get a machine, manual or CNC, get as much tooling with it as you possibly can. Tooling adds up quickly! Plus, if it is a resedential location, with single phase power, you'll need a phase converter to generate 3 phases. Or, you could install a Variable Frequency Drive, and have variable speed without adjusting belts.
One thing that I am trying to do with my machine is to have it manual and CNC. 80% of the work I do is simple, and I want to be able to manually machine a part If I choose, and use the encoder output as a DRO. This is easier said than done, unless you have a Prototrack, or a Newall, which have this feature.
Here is a dealer near to me:
They currently do not have CNC's, but do have several Bridgeports.
Hope that this helps!