I've had a M&P15 MOE Mid MAGPUL Spec for 10 yrs and I'd suggest that you do only 2 things to it.
1st, take off the long flash hider and replace it with an A2 one. It's cheap at about $10, simple to do and looks much better than the overly long one they came with.
2nd, the M&P15's trigger is a standard mil type that's mediocre at best. I swapped mine out with an Rock River Arms two stage match trigger kit. Simple to install, it will give you a great trigger at a budget price.
Optics Planet has them on sale for $90 with free shipping
HERE.