Best bet would be to get properly sized speakers, first, unless you wanna pay extra for custom mounting.

Ex. Front: DB461p

Ex. Rear: DB691

Now, if you're going the lazy route, you're gonna need more than a $500 budget.

Radio ~$215 w/ parts & install, 4 Speakers ~$250 w/ install. We're already at $465 and we haven't gotten to any sub, amp, wiring, or enclosure.