1) pics would help.
2) 20mm is plenty, and the medium duty should also be fine.
3 & 4) Motors, drives, and screws need to all work together. Imo, Gecko drives are a better option. You need to know how fast you want to cut, and how much force you want, and then work from there. 5tpi screws will limit your speed with steppers, as they'll need to spin faster at higher speeds, which is where stppers lose torque. That's one advantage servos have, is that you can spin them much faster with more torque. Multi start screws have fewer turns per inch. Ideally, for a router, you want to be around 1-2 turns per inch, if you want high speeds. Those screws tend to be much more expensive. You can use a belt reduction to get the same effect, but the screw will be spinning much faster, so you need to make sure you don't exceed the critical speed where the screws start to whip.
5) Is that a question? Should be fine to get you started.
6) BobCAD is always on special. Read through the BobCAD forum here and you may see why.