If that last example will be your runner, don't attempt to run it with those aluminium gears (drive gear + idler gear) in the rear gearbox. You'll ruin them in no time, this was the case with the batteries and motors we had back in 1992-1993 and I don't think they'll stand up any better today..
Get the main gear from the TA01-touring car spares instead, there's a plastic one-piece 74 gear available which is good. This one along with a plastic idler gear and the rear gearbox is much improved. Just my 5 cents
