Sound system, what is important?

Hello, my mother has a youth center project in Gambia. They have like a big hall where they play music but they want to have their own sound system to be able to invite dj's who don't have their own equipment. Does anyone know when u buy a sound system what kinda details are important to pay attention too, like how many watts and hz or db to have a clear sound in a hall from i guess 10-15 m width and length of 30-40 m? And maybe an example of a sound system, monitors or boxes which would be able to do that. Thanks in advance