ok don't panic... you don't have to have a crossover and to keep costs down I wouldn't worry about it...
you need at least one set of speakers, (you'd be better off with some monitors too in a perfect world - if you can't get em... just make sure the spkrs are behind the band and you'll be ok) and a pa head at minimum.
I'd check out sonic for spkr cabs = cheap but decent (made in WI!)
you could go with a mixer and power amp... or youcan get a "pa head" wich is a combo of the two... try music go round. Go to there website (
www.musicgoround.com) and you can find one near you... they have used gear.
You'll need a min of enough channels/mics to acommodate however many singers you have. Depending on how big the venue is you may wanna patch gtr, bass, and drums thru the pa for xtra vol. I'd say you need ruffly 400w to play a small to med bar. If it's outside you wanna dbl that.
I'd look for sure beta or sandl mics = under $75 and decent qty live.
sure sm57 is good or sm58 too.
there's my 2cents.
mv