It does have a built in BEC, but they are pretty much usless with anything over 12cells. The UBEC mike sells is great, or you can get one of these:
http://www.dimensionengineering.com/SportBEC.htm
So long as it can handle the number of cells/ voltage you are planning on using, it will be fine. It solders onto the + & - of the escs battery wires- right behind the plugs is best.