You can fly a FP heli with a non-heli, 4 channel transmitter.
You will need to use some kind of onboard mixing. Either the Piccoboard or a HH gyro.

If you know that you will be going CP, considder getting a decent heli capable transmitter with 5 point mixing.
The Futaba 9CH in a transmitter only package is around $280.

Buy and fly the FP model on whatever transmitter you have now. Once you have conquered the hover, you will know if the bug bite is permanent enough to consider the additional purchase of a better transmitter. You can likely sell your current transmitter online.