Is the community very active?
I can find a game pretty much anytime of the day or night. If you mean the community as the forums here, well look around, pretty active but its only about 1/4th of the actual KF players.
Is the game difficult to enjoy as a new player in the midst of established players?
Yes. You would be joining pretty late, and at this point in time, most who bought KF when it was released are all lvl6 already. You'll run into the occasional griefer but you have that in pretty much every online game. Your skill level (aim etc.) won't matter if you try to join hard or suicidal, people won't give you a chance as a lower level and you will be kicked from the server without warning. You can read their grief on the forums here as well. I personally usually give a lower perk level a few rounds to see if they atleast have some sort of idea of whats going on but the majority will simply kick you.
Playing on normal or beginner, you should for the most part have a good time but tbh if you have any skill at all then even as a lower perk these difficulties get pretty boring. Although when I reset my perks I saw so many awesome lvl 6's playing not only normal but beginner as well! As a lvl 6, playing beginner or normal is a no brainer, hardly any effort to win the game, at the same time making it harder on a lower level to enjoy the game and lvl at the same time.
Also, there are tons of servers with owners/admins who will COMPLETELY try to govern the way you play.
In time though youll learn how to play and where to play.
Is the additional content required to remain competitive?
No, the additional dlc is only character packs. They do nothing but give a little different eyecandy. Also, I believe they're only 1.99 full and on sale atm for 0.99. Pretty heavy I know.
ALSO! Buy now while its on sale! 9.99 I believe. Don't wait til it goes back up to 19.99.