Stupid question: Is a UBEC anything like a BEC? If so, why do you need one, since the Quark 125 comes with a BEC? If not, what the heck is it? And why do you need a big capacitor to go with it?
Sorry to trouble you with such elementary questions.
By the way, I'm very impressed with the factory-quality conversion of your buggy, BrianG. I'd like to do something similar myself, once I learn more about brushless motors and LiPo batteries. I'm sure I'll have more ignorant questions for you soon.