I think you will only need to buy a fight controller with GPS to get things working as you can use your Futaba radio (although if it does not have a 3pos switch, you might be limited 2 flight modes you can set-up for each flight) Other usefull addon would be a telemetry kit. The Pixhawk kit includes the buzzer, switch, power module etc…
Your idea about getting an Iris is fine as the iris is ready to go and comes with a PPM enabled radio. But this would only be recommended if you dont have any experiance with RC stuff, as building a quadcopter yourself and getting it setup is not too hard, just takes abit of time and will save you abit of money. But the Iris still is alot of fun and a nice frame.
APM2.6 or the Pixhawk?
As of now the Pixhawk and APM2.6 both run the same software which will be the case for some time. Pixhawk has a more powerful processor so will give developers more freedom to add more things so in the future it will have more features than the APM 2.6 board, but the APM2.6 board will still be supported for a long time.
Both Pixhawk and APM will need the GPS/Compass module. But I think you will need a PPM encoder with the pixhawk with your Futaba radio (but check the manual to see if your Futaba supports PPM)
Advantages of APM2.6 - it its smaller, cheaper and does not need a PPM encoder
Advantages of Pixhawk - more powerfull (might not need a PPM encoder if your radio supports PPM)
If you have any other questions, please let me know