Might as wel preface this saying that I only have 1 year (this year) experience with crab/swerve
http://www.chiefdelphi.com/media/photos/33181
This is our swerve. My brother posted images of his CAD's later in the thread.
This year we went with 1 CIM total, but for a normal year I would recommend going 1 CIM for each side (r/l) The main problem with this setup is that you have to be exact for the bevel gears, otherwise the will not mesh correctly.
We used globe motors to turn our wheel modules. I have seen other teams use FP motors, or window motors.
It is not necessary to have a computerized mill, although it does make everything easier.
We had our boxes cut out by an old CNC, then we drill pressed all the holes, then had them welded.
This year we went with 1 Joystick controlling throttle, one joystick with twist controlling translation (strafing) and rotation.
I didn't have involvement in sensors other than mounting them, but I know that we mounted a magnet onto a sensor sprocket shaft, then put the sensor right above it, no contact. This sensor then outputs a voltage that corresponds to an angle. According to our team's programmer, he has to round the values otherwise it will crash/slow down the cRIO.
If you have any more questions, please post here, or PM someone. Most people are more than willing to help