The power required to run a buffer, extractor or
vacuum would drain a normal battery in 10 minutes to the point where you wouldn't be able to restart the car.
So you would have to leave the car running, consuming fuel.
You may as well then get a small generator. It is by far the best option. In the long run will also be the most cost effective. If you run off your battery, having to jump start the car and miss a booking the money you would loose in the one job would have paid for the generator.
There are heaps for around $100 that will power a rotary and vacuum easily. Hopefully your extractor isn't heated though as they can draw a fair bit of power.