I'm getting excited for the upcoming gun show. I talked to friend of my dad's today, he told me that he saw no less than 5 mp-15's going for 550. Of course I won't be that lucky...
The M&P15 Sport is a great choice for a first AR15. Do not let the lack of forward assist (FA) and dust cover (DC) sway you from it. Among the choices you posted, its actually the best bang for the buck. Its made by a reputable and long-running company so you can expect a good rifle. The A2 front sight post is one of the tougher and more reliable setups for iron sights compared a rifle without one.
I have owned a M&P15 Sport (back in March), bought the 2013 model and can verify that it has the 1:9 barrel twist. This is the most generic civilian rifling that will handle most of the ammo you will shoot, and will better for the lighter varmint rounds you might use for hunting i.e. 40gr American Eagle 223. The 1:8 5R rifling on the pre-2013 is nice, but will not really be noticeable for new shooter.
So if you can find a Sport under $650 , buy it. $550 is even better, but I will warn you that gun shows are not good places to find cheap stuff usually. If you get lucky and find one under $600 new or used grab it right away! And If you can find the older Sport with the 1:8 rifling at your price point, that would be the best deal.
Lastly, there are a few videos on youtube that shows the reliability of the M&P15 Sport even in dusty/sandy conditions. So I would not worry too much about that. Its a great basic AR as long as you find one in good condition and lube it properly. I've never read any negative reviews on the Sport. Just know that it is a basic starter AR and does not have the other features of a more expensive milspec rifle; not really a disadvantage for your purposes. Hopefully that helps, coming from a previous Sport owner.
Good luck on hunting (figuratively) for one!