First of all, you need a way to sense the output voltage, so you need an A/D.
You then use those readings to decide if you need to increase the voltage, decrease it or keep it constant.
The same A/D reading will be used to display the voltage on the 7-segment displays.
As for the buttons, yes, you can use the interrupts if you want. You could also scan the buttons (test them periodically).
The actual adjustment can be made with resistors from the output of the decoder. Each output actually places its resistor in parallel with the lower resistor of the voltage-setting divider. Was the use of the decoder a requirement?