The M&P line has gone through some changes. At the very least, I would stay around the 2012 and above year of manufacture. They have made some improvents to the trigger (my only grip) starting in 2014 (I believe).
I ended up selling all my M&P9s and kept the 40 cal. I much prefer the M&P platform for 40 S&W over the Glock. YMMV
For comparrison, new M&Ps with standard 3-Dot sites and 2 mags can be had brand new for between $475 and $500 in several stores in my area.
