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.
"Listen to some of these guys talk, and it's like they were born from their momma's belly with a fishkeeping encyclopedia in one hand and an API kit in the other" (unrevealed).