[undertowogt1]

I think for anything decent the budget might be a little low. Even the cheap pro audio gear will cost more than 500. First of all, I would try stick with a Powered Loud Speaker, this way you do not need to buy an amp. I would try to incorporate a subwoofer as well. I do Live Sound/Video for corporate events for a living and the cheaper Pro Speakers can be very heavy on the low Mids and sound very “boxy”. A sub woofer is normally needed to fill them out if a band or music is present. With the cheaper (plastic) speaker an EQ is needed to get the speaker sounding acceptable. I think for your budget I would go for something like the link below. You can always use your music playback EQ (eg foobar 2000) to tweak the sound.
I have not heard these but the reviews look OK over all. With this package a sub is included. It is still over your budget though.

http://www.musiciansfriend.com/pro-audio/harbinger-compact-portable-pa-system

The rockville recommendation might be a good option as well, but I have not heard them.
https://www.amazon.ca/Rockville-RPG122K-12-inch-Speakers-Microphone/dp/B00M3LQLQQ/ref=sr_1_4?tag=ca-skim0x39535-20&ie=UTF8&qid=1472551837&sr=8-4&keywords=powered+speakers

[Armaegis]

I think for anything decent the budget might be a little low. Even the cheap pro audio gear will cost more than 500. First of all, I would try stick with a Powered Loud Speaker, this way you do not need to buy an amp. I would try to incorporate a subwoofer as well. I do Live Sound/Video for corporate events for a living and the cheaper Pro Speakers can be very heavy on the low Mids and sound very “boxy”. A sub woofer is normally needed to fill them out if a band or music is present. With the cheaper (plastic) speaker an EQ is needed to get the speaker sounding acceptable. I think for your budget I would go for something like the link below. You can always use your music playback EQ (eg foobar 2000) to tweak the sound.
I have not heard these but the reviews look OK over all. With this package a sub is included. It is still over your budget though.

http://www.musiciansfriend.com/pro-audio/harbinger-compact-portable-pa-system

The rockville recommendation might be a good option as well, but I have not heard them.
https://www.amazon.ca/Rockville-RPG122K-12-inch-Speakers-Microphone/dp/B00M3LQLQQ/ref=sr_1_4?tag=ca-skim0x39535-20&ie=UTF8&qid=1472551837&sr=8-4&keywords=powered+speakers

I think 500 is doable, but you have to know what you're looking for. I'll bet you can find some "breaking up the band" or "selling the bar" kind of deals on craigslist within budget.

If those Rockville units are anything like the similar looking boxes from Costco... they are gonna distort like mad as soon as you turn it up.