BEC should be built into the ESC you buy. HOWEVER, it's a good idea to run a seperate BEC... Here's why.

Most all the ESC's have linar BEC's that bleed off excess voltage(as HEAT) to give you regulated 5v(or so) there for work harder as you increase input voltage... in this case 11.1v(3's lipo) You're ESC will run hotter if you use the built in BEC.

Most built in BEC's in the smaller Amp ESC's out put 1.5A or so... this is asking alot from the built in BEC since you're driving 4 servo's and the gyro.

If something happens to the ESC during flight it will likely take the BEC with it... Auto rotation is only doable IF you have servo control, a seperate BEC will give you this control

Bottom line...I don't fly ANYTHING electric without at least a seperate 3a SWITCH MODE BEC hooked directly to the deans plug on the ESC I use 3A switch mode BEC's on all 3 of my t-rex 450's.

If you do use a seperate BEC pull the RED power wire from your ESC receiver plug... this is will disable the onboard BEC.