"I am trying to decide the best cable and guage for down the tower (no slip rings and it will be completely exposed with disconnect at bottom etc.) with this open tower."
At that voltage I would not recommend exposed wires out in the wind. They will move, flap and abrade and then they will have contact with the tower, which is a shock hazard.
I would suggest you use some sort of pipe or conduit for the wires. They can probably be quite thin, as I imagine you would only need 10 amps maximum. #12 wires would only lose about 2 volts and would be light and easy to handle. The bottom part where it does not have to be so strong can be just single wires twisting together within the enclosing conduit. This will put less weight on the upper part, and also absorb more movement.
Make some provision for short circuiting the wires on the wind turbine side of the disconnection point. It is not good practice to have it running wild, disonnected. You could have an insulation failure which could also be very dangerous.
Make sure that your tower is very well grounded.
Have fun :-)