PCM with carbon blades? Thats a good one! One has nothing to do with the other. PCM is nice to have with electrics due to the higher RF noise environments that the speed controller and electric motor create but many fly FM. My TRex is PCM, I use the Futaba 146iP

As for heading hold, its just what it sounds like, a rate gyro (non hh) dampens tail movements, say the wind blows the tail will want to weather vane, a rate gyro will slow down the drift but the heli will end up pointing in another direction. A HH (tail lock, heading lock, avcs all mean the same thing) gyro will fight to the limit of the tails ability againt the wind to stay in the same place it was. They make flying helicopters much more pleasing. The GY401 with a 9650 servo is the way to go for the tail of a trex.

As stated the smaller helis are twichy and more susceptable to wind but with proper setup they can be handled. Nitros are easier to fly but they are bigger and more intimidating, it all in where/when you want to fly.

They arent that hard to build and setup and there's a metric ton of info available online to help you. Check out http://www.trextuning.com as one example.