Booths based on a water trap system are incredibly inefficient. You'll catch a lot of pigment, but won't have much luck with vapors unless they are water soluble (e.g., alcohols, glycols, etc.). In addition, running several hundred cfm of exhaust air through water is going to send water everywhere unless you have enough volume to safely bubble the air through. Personally, I think you would be better off:
1. Getting one of the Artograph spray booths. They are a downdraft booth, which means it takes less airflow (~ 50CFM) to achieve the same purpose as a back drafted booth does (several hundred CFM depending on booth size requirements.) The Artograph booth also has an activated charcoal filter system designed to catch and adsorb organic vapors and return the cleaned air back into the room; hence no need for outside exhaust. And their prices are pretty reasonable, too.
2. Build your own booth and attach some 4" (should be large enough) flex duct to the exhaust side, open your window when you are ready to spray, run the flex duct out the window, spray, and then retrieve your ducting and close the window when you're through. Dilution from fresh air should be great enough that your neighbors won't be able to tell you're spraying (unless you're going full tilt with a super spray can, or you're painting full size auto parts in your flat). Plus the fresh air from the window will provide some additional dilution ventilation for any fugitive vapors not entrained by your booth.
Forgot to mention that the flex duct should NOT be plastic dryer hose, but the metallic variety (aluminum, I think) that can be had at most hardware stores.
Just my opinion. Hope this helps some....
Gip Winecoff