There are more than the throttle differences; the TX/RX has to be able to support CCPM swash plate movement, which combines the elevator and aileronand pitch servos into one movement which is also linked to the throttle.
So if your radio will not support heli mode I would go with a Belt CP RTF and move up to a Spektrum DX7i or DX7i at a later date.
However you can get the Belt CP v2 RTF that I linked above and a Spektrum DX6i and be good to go and still be very close to the $300 figure and after selling the RTF TX/RX you should be back to about $300, and have a radio that will do helis and planes both on the same TX, only have to add RX's to support more aircraft.The DX6i will do 10 aircraft and the DX7i will do 20 aircraft.
We will help where we can, just ask.