You have too many signals for a really small PA and you would be asking too much for it. The best bet would be to get either a package through an outlet like Musicians Friend or ZZounds, or else look for used gear.
The Passports are great for solo acts or small groups - two, maybe three people in a small area. But they top out at about 250 watts. What you're talking about is more on the order of a thousand watt system at a minimum. If you want any low end you can double the power requirements in a hurry. New systems like that are not going to be had in your price range.
Maybe think about this for an interim setup - Let's assume the keyboard and bass player already have stage amps. Now look at a system like a large Passport - about $ 850 new - to use for vocals and acoustic instruments. Throw a LDC or a boundary mic in front of the drum kit to pick up some ambient percussion. Use a small mixer (ebay) if you need to get a submix going and use the Passport as an amp.
It will work OK for vocals and so forth, as long as the room is small. In a really small room (store or coffee shop) you can place each speaker to fire directly - point blank - into a wall on the right and left. The signal will bounce and the bleed will act as a bit of monitor signal for you. You can crank it without hurting anyone sitting nearby and get your overall sound level up a bit.
When you are ready for more power, you can still use the Passport as a supplemental system or as a pretty useful stage monitor setup. You won't outgrow it; you'll just use it for different stuff.
But $300 - 500 isn't going to do it.