I would say the first thing you need to look at was what file format your customers would most likely be supplying you. For instance (just an example) if its SolidWorks you might want to look into getting a basic seat of SolidWorks and look at one of the gold partner CAM products. most CAM package's are on par for mill functionality. where i believe most people have issues is with file translation. so if you have an idea of the CAD files then you can look at CAM systems that play nice with it. Just MHO.