Best bet- buy lots of pinions from 15-22 tooth, that way you can have a play as gearing/temp is a tricky thing to get right if you're not all that experienced (including me). As for the gear locker:-
You will have to wait until tommorrow to order it, but to fit it is simple; remove shifter fork and dog thingy, then push this into the face of the 2nd gear, and reassemble the tranny as normal.
If you want the idlers you will only really need one, but you can use two if you still want to use both gears in the future.