As a band your first priority should be your stage sound. As I have said earlier, if you produce a good stage sound, you can take that sound everywhere and sound good.
therefore the first PA gear you need is a good stage Monitor system. A good monitor system requires a board or mixer head with an amp or amps. a good equalizer and speakers. You'll want to get at least 4. And get ones that come with pole mounts. I'll explain why in a minute. With your monitor system in your rehearsal space, living room or garage, work on your stage mix. This is what you'll be hearing on stage, and a big part of your overall sound. Remember to only put in it what you need into the monitors, ie vocals. Try and Position the instrument amps so that (1) their players can hear themselves well, and (2) so the rest of the band can hear them well enough for reference. Doing this keeps the monitors as clean as possible and free of un-necessary noise. Remember the monitors are for reference and vocals and only those things that NEED re-enforcing. The big mix is for the house PA.
This will take some co-operative effort by everyone in the band, and even a bit of ego taming. No one member of the band is more important to the mix than another if it's going to sound good overall. and remember, No-one gets louder than the drummer.
And while I'm on monitor mixes, when was it universaly accepted that Keyboardists get a free ride?. Everyone else has to bring an amp. Even the Bassist, and his amp is usually heavy. There are tons of versatile keyboard amps out there today, and the keyboardist should get one if they are going to join a band. No one else in the band gets to use the monitor system as their personal amplifier and it's not fair that the keys assume it's an ok plan for them either.
Once you achieve a good stage sound you can keep it and take it anywhere. All that will change is the EQ. Since every room and stage is different, a new EQ curve needs to be drawn. But since the instruments and singers and mics are the same, their levels can stay the same.
Now if you really get a good mix and sound, and Really want to keep it. Then get a stage splitter. This allows the house sound to still have all the control he needs from each mic and instrument, and allows you to keep your well rehearsed mix on stage.
Now I know this isn't the way it's normally done, (I'm a sound company and I work with lots of different bands, and normally I supply everything, monitors, mics, house PA etc.) but if your a band that travels with your own sound and even your own sound guy, then this scenario is ideal.
What if your playing a small room with no house PA and no sound guy?
Well your already set. Just take two of your monitors and mount them on speaker stands and spin them out into the room. (you remembered to get monitors with pole mounts right?)
If it's indeed a small room then your living room mix should kick butt.