What is the best way to record audio in a room for hours at a time?

I have been put in charge of setting up a way to record audio in the lobby of a pain clinic. There will be a sign, letting people know that their conversations are being recorded. There are times where patients become very hard to deal with when they aren't prescribed what they want. Especially seekers! We currently have a Sentinel-1 video surveillance system which does NOT support any kind of sound. Looking to do this on a budget, but not miss out on important conversation.