Reply to Sound system advice ??

This is likely a question asked before - but I was not able to find it ....

I am new to DJing, starting with a ballroom dance club. I have been doing it for a few months - the audience seems to like my music selection. In my usual venue we have an amp & sound system (12 speakers along both long walls). On occasion we go somewhere else. My question is in two parts:

First: Active speakers or an amp and passive speakers?

Second: Since we take no money (it is all for charity - but we buy our own gear ) and if I go with active speakers - how much better are the higher priced speakers? I saw a set - 2 15" active speakers, cases, speaker stands - for about $500.00 (which is in the budget range). Should I spend the money on speakers only (I am sure I am paying a little for the extras).

OK, a third question: I read in a number of places that a subwoofer is critical for better sound. I expect to get one - and again - what price point for something that works fairly well. I am also considering powered speakers & an amp & passive sub.

Thank you for any insight you may share.