For a 'customized' arduino consuming less energy making use of Attiny85 vs PCB Fabrication?
An attiny will be fine for a single servo and adc sensor with the Arduino firmware. The things an attiny can't do that an atmega can has to do mostly with code space and amount of timers or interrupts. Your project is very small and not affected by this limitation. 

As for developing a custom Arduino pcb, it's not that complicated. An Arduino is essentially just a breakout board for the microcontroller, with a regulator and usb to serial circuit attached,  neither which are 100% required to be on board. Look at various bare bones Arduino which are no more than the microcontroller and a few passives.