RE: Futaba 2PH 2-Channel 75Mhz
The radio set will come with a new receiver, so just remove existing receiver and fit the new one. You'll need to set it up on the bench before using it though- channel normal/reverse, idle point of engine relative to position on throttle (let go of throttle and car should be still just idling) and also brake point, channel EPA, fail safe.
Is the 3PH a AM set? Won't it be on the same channels as kiddies RC toys (sorry dunno about American channels/frequencies)
Personally I would buy from the higher range with a fail safe built in. I know the 3PK has one, bit much :-) how about 3PM?
If you want to keep existing servos, don't get a HRS system (they require digital servos) standard FM or FM-PCM will be fine.