If your going to do anything more than a trainer you will eventually want dual rates and exponential. This means you will at least need 6EXA, but for a little more you can pick up a 7c, which has timers, more mixes etc... So to answer your question, spending money on a 7c is plenty worth while if you are going to be in the hobby for a while, a 9c is even better, but unless you need the channels or dual elevator servos, you will be fine without it.

I have a 9c, and while I don't need everything it does I found the throttle idle-down and cutout features very handy.

schu