RE: Caldera 10E
its a big difference between 3s and 2s on the caldera!
the extra voltage means the motor will spin about 3000rpms faster than on a 2s lipo.
honestly, going from a 6cell nimh to a 2s lipo will be a big improvement. and for a boy, i would think 2s lipo will be plenty. expect the truck to do 35-40mph on 2s lipo.
and if you do go 3s lipo, you have to re-wire the stock fan to run of the Reciever's 3rd channel.