First; don't count your receiver pack into the cell count for the ESC power, only the main batteries, which you have 14.
A UBEC is simply a switching voltage regulator that takes your main 14 cell pack voltage and converts it to 6v for the receiver. The fact that it's switching makes it very efficient and it is very light as small compared to a receiver pack. And since it uses your main batteries, you don't charge it.
More information:
BEC/UBEC information
For a best match with 14 cells, I'd go with an 8XL.
Try this motor selector to get an idea of your options for the motor:
Motor Selector Calculator