What he said.  If you are using a true water based, solvent free paint (please note: not all acrylics are solvent free), then you don't have any fumes to evacuate, rather you have "paint dust" which you will want to capture (otherwise the room will get a coating on every horizontal surface after a few paint sessions).  This can be done with proper filter media as part of your booth's air handling.

Alternatively can you vent though a window insert that you can take in and out as need be?