for a flat top AR you're gonna want
at the least XHigh rings. Otherwise you're probably gonna be very uncomfortable getting a cheek weld and still getting into the line of sight through the scope.
i like the Nikon P-Series rings - lots of value for your $$ there and cheaper than buying a set of high rings and a ½" Riser set.
http://a.co/2Nafwtqor something like a Burris PEPR style mount
http://a.co/4Fc8d25as far as a scope - one thing ive always considered when scope shopping and why my preference is something in the 4-12 or 4-16 type range.
you can always use a 4-12x scope on 4x power, but you can never use a 1-4x (or a 2-7x, whatever) scope on 12x power.
i'm a fan of the Vortex Diamondback lineup as a good value grade scope. Nice clean glass and they seem to hold true when you're clicking around up and down for elevation adjustments. Branding aside, i strongly suggest you start shopping in the at least $200 price point for current model scopes. Generally speaking much under that price point and the glass starts to get dark and the quality of the internals falls off quite quickly.
Whatever you do stay away from the budget brand scopes (BARFSKA, Centerpoint, BSA, NCSTAR, modern tasco, current production SIMMONS, etc) that offer a lot of features at a low % of the cost of their competitors scopes. Not only do these tend to be heavier, but the quality of the internals & glass is usually questionable at best, and there's almost no QC on some of these scopes. I'm not suggesting that these companies dont get lucky and produce a good item once in a while, but your chances of getting a piece of pure sh!t are much higher when you slide under the $100 price point.
head over to a big box store if you have one close (cabela's, bass pro, etc) and ask one of the folk working the optics counter to let you look through a few scopes outside in the light of day with their viewing stock. Dont just look through them inside under retail lights. Get em out into the real world and then go from there
when you do this, just for giggles have them bring something like a VXIII out there with you. just for comparison.
just my $0.02 USD on optics.