How should I set up the pa for my band? What equipment should we buy?

[TheLastKilroy]

I have recently started a small band of 4 people. We started realizing that the drums were overpowering everyone so we decided to invest in a pa system of sorts. We have two relatively large speakers for the amplification from my friend's dad (they are from the 80's). So I've asked a few people and from what I understand we need a mixer, and an amplifier to make the speakers louder. This PA is meant for a small practice space and possibly small gigs. We think that we have an amplifier but we need to buy at least a 6 channel mixer. We found a mixer for 50$ which we want to buy, but I want to clarify that this is the equipment that would probably be needed. When we do have the equipment, do we connect every instrument to the mixer (apart from drums, because its a small practice space) through from each respective amplifier or do we have to mic every amp (ie. bass, guitar). I am curious because i don't want to spend money on something that will not work or is not needed. Any help please.