I would shoot it first if possible. 40 S&W is not for everyone. I am not a fan of 40. The husband tried the M&P 40 and disliked it too. He said that it was too snappy. I recently shot a Springfield XD in 40 and while it was not the axis of evil like other polymer 40's I have shot, I did notice that it took longer for pick up shots due to recoil bouncing the sights around. Me, I would rather have a 9mm or 45 acp.
I would find out what year the gun was made in. There have been some noticeable changes the past few years in the M&P line. But if you like 40 S&W, and the gun shoots nicely, I say go for it. The trigger alone is about $40. Sights are generally around $100. I don't know what the gun light or the holster go for. Can you get a used 40 and then add on all the add on's for less money? Will you use the add on's?