I looked of some older posts and did not see any info I use,that may be cuz I'm still really new to ARs.

What I need to know is , what are some things I can do to help my gun (Post-ban Bushmaster A2 16")work better and last longer?

I have some samll stuff like: an Ergo grip and I just installed new sites that I am very happy with.

What parts should I stock up on? That is if I even need to.

There is not much you can do to make it work better or last longer.  You can change the look/style with accessories and replacement parts but it won't make it better.

May I suggest that you get plenty of 20 and 30 round mags (GI, Thermold or Orlite but no steel) and as much ammo as your wallet can support.  Take it all out to the range and enjoy shooting the snot out of it as often as you can.  

Thats what I do with mine[:D]