A transmitter with exponential is a good thing to have if you are using 3-D control throws, rudder to elevator and rudder to aileron mixing is nice but not essential. If you use 3D control throws without exponential, you are going to have a plane that is very touchy in level flight.
Remember, you don't have to set the plane up with 3D control surface throws. In that case, a basic 4-channel Xmitter will work just fine.