Has anyone ever used the cheaper strobe-lighting kits with the umbrellas on ebay?

40 watt seconds is too few. Look to get at least 100w/s strobes. (thats each head) and even then you may find yourself struggling to get a fast enough shutter speed. I would recommend each head be around 160 to 200w/s for single person portraits. The advantage with those is that you can actually use modifiers and not be out of light.