Have you made any progress with this? What a great project. I think the Auriga would be perfect for this. 5 slots for sensors (PIR motion sensor and ultrasonic would be a must, maybe a compass sensor) and some built in (gyro, 2 light if you can relocate the LEDs, audio, temp) as well as the buzzer (could probably do some pretty convincing R2 without much fuss, you can select notes and note length very easily). Maybe drive his existing motors/wheels with a motor port and run an encoder to rotate and spin his head (and detect people and objects). Could even put a high power flashlight LED or laser in there too and get some interesting effects. "Help me, Eboutwell. You're my only hope"
The ultrasonic sensor is already pretty reminiscent of part of R2s head, might need to drill an extra hole or two in the dome though!