If you are looking for traditional battle sights, Magpuls are a good compromise of cost and quality. If you have a flat top upper with fixed front sight post, add flip up rear Magpul and you can get started.
If your upper has a FF tube with a Picatinny rail that is the same height us the upper receiver, a Magpul flip up front sight matched with the flip up rear is good.
Just don't attach a Magpul front site on a gas block, it will melt.
A set of Magpuls can be had for ~$80.
Lots of scopes, red dot sites for any budget.