I am trying to decide what equipment to purchase and am wondering how important minimum aperture is for a lens. I understand that f/2.8 is preferable in low light conditions but in low light conditions I will be using strobes. As you know, there is a big price difference between f/2.8 and f/3.5 or f/4 and the money saved on an f/4 lens can be put to lighting and other equipment.
If you are going to use the lens just for real estate photography, then there is not much point paying for a f/2.8 lens because when your lens is set at f/2.8, it has a relatively shallow depth of field that is much narrower than the rooms you will be shooting (not everything in the room will be in focus). When shooting real estate, you will typically shoot at f/5.6, f/7.1, or f/8, depending on the room and lighting so that everything will be in focus from a few feet to infinity. Low light is dealt with by shooting on a tripod and using a lower shutter speed to get the right exposure. You can study the details of the depth of field with calculators like the one found at http://www.dofmaster.com/dofjs.html.