[suicideneil]

Idler gear was simply to reverse the direction of rotation when using brushed motors; I've seen a custom made tranny around here & the guy did away with the idler altogether- looked rather nice really.

No specific name for the 2-speed spurs and pinions, they are just specific to nitro models & have a clutch mechanism inside that activates at high rpms to lock the 2nd smaller gear of the spur-cluster to the input shaft.

Pros:
no chance of mis-aligned gears or shifting components when trying to shift at speed.
Adjustable shift point to make the most of the motors/ engine's power band/ curve.

Cons:
manual shifting is impossible.
More parts to maintain.
No reverse generally.
Have to use mechanical brakes.

I would really question what it is you are trying to achieve and what your goals are, or even what vehicle you are contemplating buying/ converting?

If you use a BL system you dont need a 2spd kit to give you more torque at low rpms, only if you had a really heavy truck, or were building a crawler-come-basher that needed low speed & higher speeds that were selectable as required/ desired ( see: BL traxxas summits @ the traxxas forum ).