As a confirmed Debian zealot, that's what I would reccomend as the O.S. Debian is really the only "free" O.S., both in cost and philosophy. I agree with the GIMP for graphics as opposed to Photoshop; I have both on my computer, and while the GIMP is a little harder to use, it's much more flexible. As far as a GUI, I would suggest two; my avorite liteweight WM is XFCE, it's fast and not overburdened with addons, IceWM is also very capable. MS Word replacement could only be Openoffice 1.01; it runs very stably on my Debian 3.0 (Woody) install, setting up my printer was not much of a chore, and, as with all open source projects, it's continually improving. A possible alternative would be AbiWord 1.0.
CAD is a lot harder; as I've mentioned before, the only ongoing Linux alternative to AutoCAD is LinuxCAD, and it's not open source. I'm not sure if you could get AutoCAD to run in WINE, but if you did, I don't think you'd be very happy with it.
The suggestions that I've made are based on capability of the various apps, my personal experience with them and the fact that, with one glaring exception (LinuxCAD), they are open source alternatives to Microsoft equivilants; to me, it's much more significant to create a viable system that's running as much open source sotware as possible, rather than using a Linux-based system to run closed source software.