If you use a Elmod you may need to hook it up to a better speed control like the Sabertooth from Dimension Engineering and you can then upgrade your motors if you like.
I am currently playing with a SLU and a Sabertooth unit so my 1/6 tanks will have all the tank functions and a good speed control that will take 12 Volt and give you one stick differentail drive.
The SLU use a external speed control that i will replace with a Sabertooth ESC, so it should work for my setup.

You can find on Youtube a builder that used a HL RX 18 system and gears on a 1/6 PZ2 model and a 7.2 v battery. The Dragon pz 2 is light if built stock but the Stuart is a heavy
tank and I dont think the RX 18 will last vary long and not at 12 volt.