OP, what is it that you want? Do you want a rifle that you will just plink with at the range (not that there's anything wrong with that), or do you want a rifle that you can trust your life to ('cuz zombies)? If the former, pretty much anything out there will serve. If the latter, don't skimp by buying the most skinflint parts out there. The BCG is the most important part, and it pays to know that it's from a reputable mfg. Next would be the barrel.
And a $550 Sport is as reliable as anything out there. Just my .02
I think one of the key things here is taht the OP doesn't know yet what he wants or likes.
I made a mistake when I bought my first "new style" AR. Prior to it, all I'd ever known was A2s. I had an A2 HBar from colt and a Bushmaster CMP gun.
My mistake was a LMT MRP.
The things I learned from that mistake:
1) I hate quad rails.
2) In shooting the CMP gun, I'd become a trigger snob
3) While taking my first carbine course, I learned that I really wanted an ambi selector
4) I learned that I like my rifles to be more accurate rather than mil spec. (stainless vs chrome lined)
5) I like my rifles to be light for a given accuracy - no M204 cuts or other things that hurt accuracy.
6) I don't really care if the stock telescopes, so why put in a crappy pinned stock. I became a fan of the Vltor rifle modstock.
7) Red dots are nice and all, but I'll take a 1-4 scope any day. Just as fast, and more accurate at distance. I also really like shooting iron sights.
Here is a pic of my "mistake" rifle. What I had figured out at this point was the sight, the ergo grip and the ambi selector. I first got it in 2005 or 2006. I'd guess this pic is from later because I've settled on the items mentioned above that I like. I was still holding my head erect on the stock, like I was shooting a target gun, which you can see by the rearward location of the scope.
The only reason I didn't get hurt with this "mistake" is that I sold it in 2008 after BHO was elected president and LMT was backordered a year.
I only sold the upper, as the lower was a pre-ban SBR. So it morphed into something else.
This was the next iteration:
Then this:
Reconfigured for a carbine class. I finally found a comfortable telescoping stock and the aimpoint went back on since we weren't going to be shooting past 25 yards. For speed, I took off the can and put on a good performing POF brake I had on hand.
Then I decided to start from scratch. And ended up with this. Its definitely NOT mil spec. But it shoots like a dream and weighs 5 lbs.
I fully acknowledge that it looks like an abortion. but it has everything I want.
geissele trigger, ambi selector, ambi mag release, light weight, accurate (lothar walther bbl), and light shooting.
By the time I built this, I'd really grown to appreciate a low mass operating system with an adjustable gas block.
This makes for a better shooting gun, but one that is less tolerant of dirt and scuzz. Since this is a toy. Reliability was sacrificed for speed and accuracy.
This is truly an extraordinary rifle. It weighs 6.2 lbs with optic, shoots 1 moa until the bbl heats up, shoots fast, handles great, and didn't require a tax stamp, But it is UGGGGGGLLLYYYY. Oh well.
Back to the original topic. Start with the Sport, then let your imagination take you where you want. My guess is that 20 years from now, you will still have the M&P. I still have my Colt AR15A2 HBar and my CMP Bushmaster.
Don