The reduction will depend on if there is a further reduction in the differential as this will affect the speed also, I take one step at a time first fit your differential with the sprockets mounted once you have this done then the rest is simple, the diff is readily available off the shelf
http://www.ebay.co.uk/itm/like/39126...7297426&crdt=0 fitting this is simple you then need to attach this to the sprockets once you have mounted the sprockets and diff then go onto the transmission.
The R2 transmission is quite good
http://store.rc4wd.com/R2-Heavy-Duty...ion_p_850.html you also have the ability to alter the gearing via a pinion to suit whatever gearing you want.
As for fitting a break to the diff this can be left until you have the chassis running then see if you really need a break on the diff to help with steering.
Have you tested the plastic to see if its brittle.