1) Some teams were experimenting with them. I believe experiments were going well but you would have to find the specific teams and ask. I think they were opting for the 36v versions as 48v controllers are more difficult to come by.

2) Ragebridge 2 jumps to mind as a fairly low cost option for them. Your other options include, wotty (made by ian of bigger brother / orte), vantec, roboteq. I think there was also another brushed controller that was a custom made job kicking around on the forum but the name escapes me.

3) If you are planning on using the bearings in the wheels and having dead axles then bolts with some tube collars on them should work to hold them on. Depending on what sprockets you plan to use you could remove the bolts in the wheels and use the same holes to bolt it all together with some longer bolts and the collars on the bolts. This would also allow you to easily drill out matching holes in the sprockets.