1) I use a V-belt on PP3D same as ironside did. B-profile. I like them because as you say they do slip but they provide a very good simple way of providing power to the weapon. No one uses a wheel pressed against a weapon because it's not a very effective method of transferring power. The best way to think about it is, with a belt profile, I have contact right around the inner surface of the belt when it's touching the pulleys. Lets assume the pulleys are the same, that's essentially a full pulleys worth of contact to transfer power. A wheel touching a disc or shaft has a small contact area where the two touch. That's it. You simply can't transfer power quickly enough through such a small area.

2) 90mm diameter titanium bar - 230mm in length. Complete overkill but it's the one part of the robot I've never had an issue with. Plus I got it cheap on ebay. You could calculate optimum shaft dia etc but quite simply, go as big as your design will allow.

3) Bearings are top and bottom of the shaft with the disc bolted onto the underside. 80mm bore ball bearings. Nothing complex about them. Cost around £25 each. You can also use bushings but you will need more energy to get the shaft moving and these are not great for high speed applications.

You can see the various bits of PP3Ds assembly on the website http://pp3d.weebly.com/

I put various CAD images through the stages, the various parts of the bot and some overall pics throughout.