The Primary Arms Micro Dot MJ mentioned is a great choice for the money. I have a couple of the similar Bushnell TRS-25 red dots that have held up well also but they're probably not as robust as the Primary Arms and certainly not as the Aimpoint Micro.
Lots of good choices in scopes, but harder to come in under $200. I got at Nikon Prostaff 3-9x40 recently that's done real well so far and was on sale for $100. Cheap but I think it's gonna do just fine.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek