RE: need input about the Walkera Dragonfly 36
You should get a 3000 kv brushless motor and a 12t pinion. I use my JGF 400DF and CC35 esc, have about 400 flights on the motor and it's like new. The 2.3mm shaft 400DF allows you to use Revco pinions, a lifetime gear. You will need a castle 25 or 35 with the 400DF unless you can find a cheaper esc that will soft start it.
A cheaper heavier combo will be an Align 430L 3150 and BL-35G esc. The 35G esc has a good BEC for 4 servos. Use an 11T pinion with the 430L 3150. A 430L 3550 will be too fast for the 36, in my opinion. Don't go over 2500 head speed on the plastic head.
And since you are getting a DX6i, get an AR6000 for the 36. Don't use the stock radio. Probability is high that it will lock up and cause a dangerous condition. You are warned.
CH