Okay, guys! ^^ I am currently considering
]XtremeWorlds. It is not open source, but has many features and some scripting support, which allows for many extra things that I already got in mind.
The PvP works very easily. I can set a minimum level for PvP (PK'ing, they call it), and I can decide which areas allow it. By default, if you kill the player, you get their items and some experience.
My second option as engine would be
X-Realms. It is very similar to XtremeWorlds (same source), but with more scripting support and less default features.
Before going to sleep, I am going to contemplate the graphics issue and decide what to do. I will also be making a list of current priorities, so that those interested in helping know what they can do right now.
Again, thank you!