Need help with optics on LaRue Stealth

I have a LaRue 18" barrel Stealth and am looking for ideas on what optic to go with. Do not want to spend a fortune and the budget is maxed out at $500 and that’s got to include optic mount.

The range I shoot at goes out to 600 yards.

This is my only AR15 so I need a versatile optic.

What should I be looking at and why??