I am in your position and I can tell you this: The engine provides most basic features to create a 2D 'MMO'. But if you are looking to do something specific, you'll have to edit source or hire someone to do so.
There is a lack of documentation; though I think it is more then off-set by a friendly community that doesn't mind answering the same questions over and over again.
My personal suggestion would to just start playing around with things, trial and error is a very effective way to learn new skills. ( I spent 4 hours editing 1 piece of GUI cause I suck at it, but still felt rewarding after i figured it out on my own after many failed attempts. Start a small game, don't go big, it will most likely not live very long and you will learn all you need for your next one. Most people here have spent years on their games and most never see the light of day. Enjoy the process or hire people to do it.