As it's a FAAST transmitter you shouldn't need to use a seperate receiver but you'll only have access to the channels that the P2 stock receiver provides. Clearly if you want to utilise those couple of extras you'll need to fit a 10 channel receiver.
There are guides for various Futaba tx/rx...
You could just use a mounting kit to allow you to put the H3-3D on a standard P2 body
http://www.quadcopters.co.uk/zenmuse-h3-3d-mounting-bracket-phantom-2-old-1418-p.asp
Just out of interest why are you looking at replacing the Motors, ESCs etc?
In the Assist software, check to see what the switches are doing. Go to the 'Basic' screen and you should be able to toggle them watching what they are doing. The control mode is the one you are particularly intersted in.
One thing to check that caused me no end of problems (until I discovered it) is take top top off and gently flex the motors and check for any cracks around their mounting screws.
it's also just worth removing the GPS from the top and making sure the ceramic antenna surround isn't cracked...
I've had mine 10 months, too many flights to remember.
Only one major incident, blade failure at 120m altitude. Luckily it fell into a hawthorn hedge. Body shell was damaged with one motor mount cracked.
Early on I replaced the stock Receiver / Transmitter with a Futaba 8J
I'm sure most of our European members have already seen this. Not the most responsible of uses!
http://screamer.deadspin.com/havoc-at-serbia-albania-as-match-interrupted-by-flag-ca-1646284088