I purchased an SE kit off of eBay also and got the 4050kv motor. I asked around and pretty much was told not to use it at all, you would have to run a 9t pinion on it to keep the headspead where you need it and I was told the 9t is harder to come by. I then e-mailed Align and was told DO NOT USE, return for exchange. I sent it back and got the 3550kv silver end cap version. I would not spend extra on the 4050kv. The 3550kv is the motor that Align intended to be used in the SE and that is what I would think to do.
Trey