#1
my band is using a ****ty crate amp for the mic, but you can barely hear it and it overdrives.

Im looking for an amp that i can use for the mic, or i might just buy a PA.

i dont have a price range, but up to 600 USD is good
#2
600 will get you a cheap PA system for the whole band to use, if you buy enough mics later on.

Get a mixer and a pair of active speakers. That's enough for club gigging. I can't point to a specific brand, really, but I advise to save up a little more. Or get one speaker for rehearsals and get the second one later for gigs, once you have the money.
#4
Quote by grifff
Do you plug your amp into the pa, or just the instrument into the pa?

It depends.

Microphones go into the PA.
Instruments get a microphone stuck in front of them.

Drums, vocals and acoustic guitars get mics. Electric guitars and basses are played through their amps, and each amp gets a mic.
It basically takes the sound of your amp and disperses it throughout the entire venue.