1. Sorry the guy above hasn't really read you're post properly. There are a few of us on here that have used cheaper controllers with success, it's mostly just a case of being careful what you buy. The amps quoted for the linked controllers will be for peak amperage, not continuous. They're also very expensive for what they are, if your after some cheap controllers to have a go with I can link stuff that's a lot cheaper to run a pair of drills.

2. You could write pages on mixing. It's basically if you had each motor on a separate stick, you'd push both forward for forward, back for back, and one forward one back to spin the robot one way or t'other. By using mixing, you can put fwd/bk and turing onto one stick so when you push left, the transmitter realises you want left and spins the motors opposite directions. Mix fuctions are available on transmitters or dual speed controllers, so be careful what you buy. Ask if you need more info.

3. Yes you can use drill batteries, but they take forever to charge and weight a ton. Learn about Lipo's (again can explain if needed) and get one of those. They're only a tenner.

4. cc3d and naze32 are flight controllers for quad copters with gyros n stuff built in. They work in 3D and their functionality doesn't really give you the option to reverse motors for tank style steering. They're designed to keep a 250 race quad level. Getting the controller for a robot is just a lot simpler.

Anything else just yell.