Thanks you Scarlet for the answer to my questions, and for these wise words about game dev. I appreciate you being frank with my lack of skill !
Indeed, I should and will stay away from MMO. I've always wanted to make my own MMO and I like to learn the hard way, that's why I bought the source, and it's very simple, so quite easy to understand so far, but I will follow your advises and start with something simple.
I have learned PHP and C++ at school, and was the best of my school, there I've made a pong game, and a network card base game for 4 players with IA. That's my close to zero exp. Then I moved to developping apps and complexe websites. So I am really unexperienced in game dev.
I will tweak my project, to make it a solo RPG first. And, if I enjoy my time, and realize I'm getting good, I'll buy this engine, who seems really great, and quite cheap.