I have an M&P 9c and M&P .45 full sized.
The 'must have' change I did was what I had to have. I don't believe it is a must for all people.
I had to have the Apex DCEAK trigger/sear setup installed, with Apex's RAM device in the 9c [my .45 has a thumb safety so the RAM won't work]. This removed ALL grit/creep from the trigger, except that flex I felt from the actual plastic of the trigger moving.
Other than that I'd say ammo.
I also concur that a 9mm or .22lr is a better first gun, or a better frequent shooter. That said I don't think the .45acp is as bad of a first gun as the .40S&W. The .45acp may cost more to shoot, but my experience is that the .40S&W round has the worst feeling recoil of the three [9/40/45] and it caused me to develop a bad flinch that took me 5 years to get rid of.