I would think that the best system for this company would be the one that minimizes loss of the company's existing assets (map database, artwork, etc.) If they are one of those companies that has physical paper assets rather than electronic ones then your problem becomes much more difficult. Now you need a way to get the existing assets into electronic form (which implies a scanner of some sort). Scanners typically generate raster data. Adobe's Photoshop is the gold standard for professional raster editing. Note, however, much of typical map editing that would be done in a map house is vector in nature (overlay this line in this area as roads, this area as rivers, and so on), leading to the problem of raster-to-vector conversion.
Once everything is in a nice editable form you come back to the problem of doing the editing and producing the maps. Vector programs such as Adobe's Illustrator and GIS programs such ArcGIS are excellent for handling vector data. Illustrator is easier to use for many folks, but a GIS program will give you more flexibility if you're working with real-world data. The physical production of prototypes can be handled by a large-format printer with final press output being handled from the master artwork files.
With regards to PC or Mac, you will most likely find that the same software packages are available for both. This means that you are best served by recommending the system that you and your client feel most comfortable using and supporting. You'll want to be sure to get the 64-bit OS and software versions along with lots of memory in each machine (8 GB minimum). On the video card front, lots of memory is useful for professional-type packages, with a minimum of 512MB on the card being as low as I would go these days. I've had many good experiences in the past with NVIDIA-based cards, but the ATI-based ones are slightly faster these days and the drivers are comparable for basic OpenGL usage.
Note that there are open-source versions of most of these packages (raster art = GIMP, vector art = InkScape, GIS = QGIS or GRASS). They can be highly effective, but it can be harder to find experts available for them. Certainly their cost of acquisition is much lower than the proprietary software (expect to pay more for software licenses per machine for proprietary software than for the machine hardware itself).
I've had to make a lot of guesses about the kind of things that you want to do here, but I hope that this discussion was useful.