The 80 is four feet long, so you should have no issues keeping the less aggressive species of Mbuna. I would stay away from Kenyi, Auratus, and Johanni. Using aragonite sand or shells works well to maintain pH at a constant without having to buffer your water. As mentioned, landscape supply companies or quarries are a great source for cheap rock, but sure and test it prior to placing in your aquarium. I would also recommend staying away from a planted tank with Mbuna. Mbuna are herbivores and most types of plant are viewed as a treat by them. You can occasionally get away with some, such as anubias.