PA system up to 500 people

[dickiefunk]

I would look at a secondhand Nexo or KV2 audio rig.
Something like a pair of Nexo PS10's/12's over LS600/LS1000 subs + amp/controller. These go within your budget fairly regularly secondhand.

Alternatively a pair of KV2 EX10/EX12 over their EX1.5 or EX2.2 subs is a great rig.

I have been successfully using a pair of Yamaha DXR15's + QSC KW181 subs for similar sized events and these have been great. Yamaha updated the DXR range last year to the DXR II range which supposedly have great clarity in the mids and highs. Also, the Yamaha DXS18xlf and new QSC KS118 subs outperform the older KW181's!
I know a few people prefer the Yamaha DSR112 over subs for louder gigs.

RCF, FBT and JBL also have some good options.