I built my 8 year old a Monster GT. Accommodating the motor and esc were easy, but I did have to get a bit creative with the tranny to lock it into first gear and remain strong enough to take the torque of the brushless. After having gotten it built, the only things I've had to replace are rod ends. It's a VERY tough setup, although the tires are on the big side.

To keep things affordable, I went with a Mamba Max controller, a Feigao 8XL, and a 4S2P 8Ah maxamps pack. It runs for it seems like forever, with the conservative gearing. The whole things acts like a fast crawler.