Another thing, outside of what has already been said, your lens appears to be a short focal length lens as well. Most of the shots show some very strong distortion and because of it are not conducive to portrait work.
And I am not going to "judge" you on the "rental" thing, but it is pretty petty to use something and then plan to return it after use.
The concept of flash photography is a whole different world. You really need something you can play with and learn with. The sunpak's are pretty cheap and would be a good way to get your feet wet in flash. As others have mentioned, find something that you can afford that will let you get your flash out of the hotshoe. Until you can do that, your not really going to like what you can produce. It is passable, but not something you will be happy with once you get the hang of using off camera flash.