I have that same rifle, and a .22 upper on the same lower (so 2 overall). It's fine for the $, and yes, it should be a DI rifle, not a piston upper.
Very first thing, is give it a good going over and cleaning. Lubing it properly is something many skip, and then have issues if they run the rifle hard (like 3 or 400 rapid rounds hard sort of work/training). Good video on AR15 lubrication -
https://www.youtube.com/watch?v=rzVr7JXVxH4 . A lot of guys will clean their AR, but not lube it very well, there are a few important places you NEED to have proper lube, lots of products, but the Walmart hi temp bearing grease as in the vid has always worked best IMO. 8:30 or so, most important part of the vid/lubrication starts there, but watch the whole thing if you haven't taken one apart before.
Get yourself a good sling as well Rip, standard slings are fine, BlueForce makes one of the better higher end slings IMO if you want something like that. More magazines as well, 10+ would be better than 4, 4 isn't nearly enough, get them while they are cheaper (now). Function test all your magazines, load them to capacity, and make sure they will insert and stay in the mag well with the bolt closed. Also, some sort of rig to cart said magazines about in if you plan on shooting it often or training with it. Piles of good options out there for that which don't cost very much.
Other doodads to your hearts content - Some kind of better than stock Colt back up or flip up rear iron sight will be a good idea IMO. Tritium insert front post as well. Optics too if you plan on that- depending on range/use, as well as budget, many many options.
