To add to the above, an important thing to keep in mind when buying a ESC is that it is rated for more than the amp draw of the motor you choose to use, this is determined by and relitive to your choice of prop and battery size
example a 1750 KV motor running on a 3S lipo turning a 7X4 prop will draw about 16 amps at WOT, in this set up a 18 amp ESC will work but it's right at it's limit
now drop to a 6X3 prop and your now down to around 12 amps
same set up on a 2S and now your down to 4 to 6 amps

now the only reason you would use a seperate batt pack for your servo's and RX is if you are running more servo's than the ESC can handle or if you have a large amp ESC that doesn't have a BEC, or if your running multiple motors, more than 2 and a flight pack would be advised

I found that at first it was easier to buy motor ESC combo's that would do what I needed untill I learned enough to just start putting this with that

I hope this helps