Do you get a new gun or modify the one you own?  That would really depend on what you are  intending to do with it.  I would not take a hobby gun to a serious carbine course unless you want to make a fool out of yourself.  The real trick is getting good parts and replacing second rate extractors and springs.  That really is something that can make a huge difference in keeping your firearm up and running.  I have never seen an AR15 go down because it was dirty, most of the ones I saw go down where due to magazines being inserted improperly or an operator error.   The rest of the time it was from parts sheering or breaking.

The things that people tend to one to do with firearms is hang crap all over them, now the trend is towards getting light weight uppers to keep your weight down.   The best product line for accessorizing your AR15 are the Magpul accessories.   We sell about a half a dozen lightweight upper receivers and they all have the new MOE stock accessories.  I  personally have not taken that ride yet but I do see some advantages in it.  Most of my AR15 rifle are close quarters rifles and I have very little use for lasers or large flashlights.  I moved on to the magpul angled fore grips after not having good luck with folding grips.