The Dingley International is opposite the airport on Boundary road, a 15 minute walk to the airport. There is a new Quest on Heatherton Road, about 2ks further than the Motor Inn on Nepean Highway.
If I was he, I would just sleep at the airport, cost you nothing. Bench, tent, grass, under a wing, could sleep in the middle of the field if you wanted to.
One of the helicopter joints up near lwr dandenong road has overnight cabin type accom, The Helicopter group from memory.