I have a Futaba 6 channel and use five servos for my four channel plane. The fifth servo plugs into the #6 slot. This allows me to use one servo for each aileron, and the ailerons can be used as flaperons or coupled with the elevator in advanced mixing.

Otherwise, the other channels can be used for bomb drops, flaps, retractable landing gear,etc as you mentioned.

I think the Futaba 7 channel you mentioned looks like a good radio and is priced well.