I see what you're saying.. if you don't have a pile of parts laying around buying a new parts car might be the way to go. But remember, electronics can be had pretty cheap. Hobbyking has receivers and servos for around $10 apiece.
Depending on the weight you're going to end up with and what your gearing is going to be like, the Tornado might be ok. You're going to end up with a lot more stress on a single diff, both because it will see twice as much power and because you won't have the front wheels pulling it along. It would suck to commit to one diff and find out it didn't work (not really sure how intercheangable they are, might not be that big of a deal).