CNC is a wonderful way to do precision machining especially with 2D profiling as you mentioned. I would seriously consider 3 axis as that Z can be very useful with CNC. Again, it all depends upon the type of work you are doing.
The first question that I would ask is what materials are you planning to work with? Steel, aluminum, plastics, etc?
The second question would be what is your budget? Do you want to go the stepper route or servo route? Steppers can be less expensive. Servoes can add precision to your set up especially when backing up and restarting from a specific line of g code.
Third question is how much time do you have to dedicate to this kind of a project? It might be cheaper in the long run to keep your BP as is and look for a used mill that has already been converted. It's a buyers market and with the potential of the economy taking another dip, more deals may be forthcoming.
In May we decided that we needed a more rigid mill to handle some of our projects. We looked at various milling centers and mills. We decided to get a Bridgeport that had already been converted and was in working condition. I got lucky in finding a pair of Series II's in Southern IL for not too much.
You might also look at Tormach's and Novakon's. I believe that they're turnkey out of the crate. But you will be purchasing a Chinese sourced product there.
The benefit of purchasing something that has already been converted is the saving of time and knowing that it was done right from the get go. It can be a bit hair-raising for some folks to add CNC to an existing manual mill. It's certainly do-able but know that there are challenges.
You're also going to need to look at software. You'll need CAD, CAM, and a controller. CAD apps continually desire more powerful computers. I have one computer set up with Rhino and Alibre plus CAM. The computers on my mills just handle Mach 3 so they're pretty basic with the exception of touch screens. Bob Warfield has a good treatise on his website that covers CNC. Here's the link: http://www.cnccookbook.com
I drop in there from time to time as it's a great resource.
Hopefully I haven't confused you too much.
Good luck and let us know which way you go with it.
There's only one way to find out, might as well get started now!