Hi prof_jazz, welcome 
please let us have a photo of your new pet
I used 6V HITEC HS-325HB hacked to run continuosly, but this motors are not enough for he job they have to do
I also waste one year following this track 
(1) in the setup, with the function calibrateSensors(); you have to put the robot (each time you switch it on) exactly in the static equilibrium position in order to evaluate zero values (exept for z).
is this a little bit boring and could produce errors (if it is not perfectly perpendicular)?
If during the calibrateSensors() i'll give a tilt, the Robot will work to keep the tilt!
No No...
If you tilt the bot forward before calibrateSensors(), it will move forward and find a vertical equilibrium
This will happen only with encoder(s) implementation, look here
(2) what do You use to recharge the 12V batteries?
I ordered the battery charger shown here