Lemme try and answer a few questions at once.
As far as a CNC mill? How big of an area are you wanting to work with? I hear sears sells a little one, but i dont know about it working with metal.
Some people take a manual mill (for metal working) and put motors on the handles, so it can be used manual or by CNC. This might be a solution, but your severely limited on how big of a project you can work on.
If you looking for a large machine, or for working with a large area, i suggest you check out
http://www.joescnc.com and take a look at his designs. I built my machine based largely on information from one of his early machines. I'm currently building a modded version of one of his machines. One of his machin designs is free, and with a little thinking, you can build a CNC machine with a tablesaw and a couple hundred in online electronics ordering. JoeCNC designed a rock solid machine, and though his previous designs are all free, his latest machine plans are $100, totally worth it if your really interested in building a large format CNC, because for about $2000-$3000 you can build a 15-22k machine.
Software to run it comes in a couple of parts. You need to make parts, either 2d shapes, or 3d models. Sketchup (Free) is great for 2d models, but you need the "Pro" version to get 3d models easily (because its possible to get the 3d file, just not exactly straitforward)to the machine. Vectric.com makes several products for producing gCode (The actual code that the machine "reads") based on shapes or models. The nice thing about sketchup is that you can just select models that other people have created, and feed them to the machine. (Hello new-Bat-Mobile Pine Wood Derby)
MACH3 (Artsoft) is the other software you need. Where Sketchup is a CAD (Computer Aided Drafting) MACH3 is a CAM (Computer Aided Machining) This is the stuff that does the 'reading' and Actually controlls the machine.
About the 20cm question.......
There is no limit to thickness because with the software from vectrics, you can "slice" a model into parts for certain thicknesses of material. Meaning, if i wanted Tabula, and wanted it thicker that 1" i can "slice" it and tell my machine i will be using 3/4" material, and if i want 6" tall "cliff" in tabula, it will break it into 4 parts, that get glued one atop another. (see vectric.com)
I didnt do this with these tables because i didnt have the money for the software, but i recently landed a big job (part of the reason i didnt get this think done a while back) and i have the money for it now. I'm in the middle of building my new machine, so it might be a bit before i get the new software. The machine is important to me to get finished, because it will greatly effect my quality.
Never the less,
for the 4th time now it has been suggested that i make D&D style campain maps. I have some ideas on how to get it done, but don't know about the Return on Investment.
Would a player want a 3D campaign map?