I am a new owner of a bushmaster XM15 M4A3 carbine and i’m very excited. I am just learning about the rifle. I am a police officer and will be using my AR for work. I know some of the basics as my dept provides ARs as patrol rifles and have provided some very basic info. I would like to know how I can improve my AR. I read some of the threads about different extractors and springs and O-rings. I want to have a top notch rifle. So far I have added the surefire four sided picatinny rail mount, an EOTECH 511, a Command arms pistol grip and fore grip. I have also added a rear sight for co-witnessing and a pentagon MS 3 Xenon light. What should I do (or have done) the internal portion of the rifle, cost does not matter, this has become an obsession. Also some advice on mags, I bought an extra bushmaster mag but I’ve heard there are better. THANKS TO ANYONE WHO CAN HELP. 
this has become an obsession
Welcome.
Sounds like you are good to go so far. Internals, to me, are internals. Others will say that is wrong…
Learn it, use it, practice and practice.
Most USGI mags will suit your needs. As far as the best mags? Others will have to answer that. As long as they feed…
Congratulations on your rifle and welcome to the forum.
You will undoubtedly receive some expert advice from the forum members. I’m not an expert, however, but will suggest that you ensure that the bolt carrier keys are properly staked (you also might want to upgrade the bolt’s extractor insert and add a Crane O-ring; G&R Tactical sells the BCM upgrade kit with insert, spring and O-ring).
You will need several (many) more magazines. I use USGI mags with Magpul followers and Wolff springs. Others prefer magazines from Magpul, HK, etc. There is a sticky about mags on the AR General Discussion page.
Otherwise, train, shoot, and enjoy.