with cnc's, it is pretty much you get what you pay for. so consider it being a considerable expense. you can spend as little as a couple of thousand, to $50k and more. the options (as you can imagine like a car) can add up as well. some come with the software for CAD part design, and CAM machine control, and some you have to pay extra.
the table size will play a large role to meet your needs. many programs allow you to "tile", or machine half of the job, slide the work down, and machine the other half. so a 4' table can route an 8' job.
most machines we talk about can route wood, plastics, and non-ferrous metals.
generally a cnc will come with a router (of various sizes and brands), but a common upgrade ($$) is to a "spindle" which run quieter and last longer.
in my opinion, technical support plays a very large role in the cnc business, as the learning curve can be a little challenging. some brands of cnc have their own forum online. this is a good site with many knowledgeable folks willing to help!