If one doesn't have free time, consider half the budget for the RPi+camera, then just find the closest makerspace or kid with an interest in electronics and ask them if they would like to hook it up for you, you saved half your expense, the kid has some more experience and some money to show for it.

Same with the amplifier knob, just needs a cheap wireless microcontroller, and sufficient low pass filtering on a PWM signal, perhaps a quad op-amp IC for the filtering and the voltage buffer...