You need to be very careful when building a spray booth, because electricity and organic vapors from painting make very incompatible bedfellows. Fires are not fun. Booth walls should be of non-combustible materials, not wood, cardboard, etc. Fans should be at least brushless; that is, they have no brushes rubbing on armatures and creating sparks, and the fan has to be powerful enough to exhaust the vapors out of your apartment. CFM requirements will vary based on the dimensions of the booth and length of exhaust ducting. Don't even consider bathroom exhaust fans. Although more expensive, it may be more advantageous to you to investigate in a pre-made booth by a reputable company.
Gip Winecoff