I would just make sure that it's reliable! there are a massive list of goodies you can add but not needed to get out there and shoot and have fun after all that's what this is about. I would offer a couple of suggestions, the triggers on AR rifles for a better word suck! a rifle is only as good as it's trigger and accuracy will suffer with a crappy trigger. No need to go out and purchase a scope as iron sights work just fine, a little practice and a 10" plate at 300 yards will not be that difficult to hit! one hint though is to get a finer front sight pin of around the .030" size. Get some ammo and go shoot is the best advice I could offer, take your time and make it good practice! 10 rounds of thought out shooting is steps above and better than 100 rounds just shot. You will after some time shooting figure out what you would like to add to your rifle.
Welcome to the board, lot's of good folks on here and willing to help just don't mind the ribbing it's part of what makes this site what it is. Later,
Kirk