As others have said, talk to the PA department at the local music/guitar store.
If this is mostly for voicework/announcements/etc, the job will be easier.. Coliseum style speakers and horns might do the job. Trying to get full range and bass for music will be very very difficult and expensive.
If you're ok with a quick hack job, then scour craigslist for used gear. Old workhorse amps from QSC, Crest and Crown are the way to go and can be found for cheap. PA tops from JBL, EV, Peavey, etc should be fairly easy to find as well. Avoid Behringer and Alto brands if you can.
You do not want to have super long runs of speaker cable... 50ft at most, and it better be a thick gauge at that length. If more than that, then yeah you're looking at a 70V system which has different gear requirements and you're not likely to find much of those on the used market.