PA is an abbreviation for Public Address (system) but in the context above it means where your vocal mikes would be plugged into, and your guitars, and whatever else needs to be amplified electronically.
You can get by without the low bass - the keyboard should be able to set the tonic notes of the chords to furnish that aspect, and maybe even emulate some bass runs on his left hand.
Or, Guitar Center may have some ideas on bass keyboards, there might even be just such an animal in stock.
The three of you should be able to fill any small venue with sound and rhythm, with just a small amp. A few weeks ago I saw a reference to something with a name like 'Street' amp that you might find at the Guitar Center too. Might be a Roland brand, can't say for sure.
Get out there and play, that's fun.
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