I would definitely recommend no less than 3000W. That should power most anything you throw at it (including an extractor and other power hungry machines).
As far as brands, I highly recommend Honda. They have a good track record of making great engines, so anything in the Honda line will work great. They have different model lines, and one of the best is the EU line. I have an eu3000is and its amazing. It has an electric start, can run up to 20 hours on 3 gallons of gas, and is EXTREMELY quiet (seriously, you can start it up and stand right next to it and have a conversation without much of a problem). It'll set you back $1800 for a new one (I picked up a used one), but if extended use from a tank of gas and noise level are important for you, then there's no comparison. Some customers really appreciate that you're not running a ridiculously loud generator at 7AM on a Saturday morning, or at an office complex.
Again, thats a top of the line unit, and having an extremely quiet generator was important for me, but it might not be for you. I'd recommend anything by Honda, though.