How large is your studio? How high are the ceilings? What color are the walls, ceiling & floor? How many windows and what size are the windows? The answer to these questions will determine how many strobes and how powerful they will be. You say that you also do portraits outside. What do you use for fill light for outdoors now? I would suggest at least one strobe that is the most powerful that you can afford to serve double duty as fill light outdoors and as a main light indoors. I don't care for softboxes as they give the unnatural square highlight in the eye. I prefer octadomes or umbrellas to give a round catchlight. What ever you end up buying, make sure that all your umbrellas, softboxes, octadomes are made by the same manufacturer so that their color cast will match throughout your scene.
