You have a flat top?
If you do, a standard A2 style sight on a carrying handle will be fine if you just want iron sights. If that's what you go with, strongly consider adding a set of trijicon tritium night sights. The complete set is about $75.00, and gives round the clock targetting capabilities.
You know....for zombies.
If you do go with a red dot, there are a ton of them on the market that are quite serviceable for under $100.00
There are several existing threads with a variety of brand names and models. If you get anything with "Aimpoint", "EOtech" or "Trijicon" on the side you are paying a big "tactical tax" as well as buying a product that has capabilities a civilian will just never need....like night vision compatibility, for instance.
Tasco, Bushnell, Redfield, Walther etc, etc, etc all make perfectly serviceable red dots in the $100 range.