I'm not all to familiar with the hdx300 I'm assuming 3800kv. I know many people have gone with that before... 18A should be fine, I am guessing it is the sonix's 18a esc?

For good info on that setup check out this website.
http://www.helifreak.com/viewtopic.php?t=17482

I think most people go for the park 370, either the 4100kv or the 3600kv and the eflite 20a esc, it is more money though
4100kv
http://www.horizonhobby.com/Products...rodID=EFLM1000
3600kv
http://www.horizonhobby.com/Products...rodID=EFLM1010
20A esc
http://www.horizonhobby.com/Products...rodID=EFLA311B

Also I am not completely sure on this one, but I think on the outrunners the bottom of the motor spins with the shaft instead of just the shaft spinning. If this is the case... then I don't think an outrunner would work... if not, let me know what the real difference is between inrunner and outrunner =)

Hopefully that gives you a bit of info... and maybe someone with some personal experience with that particular setup can reply =)