In addition to the above mentioned Realm Crafter
, take a look at Hero Engine
Realm Crafter will allow you to easily create a basic online RPG to play with a few friends, but probably isn't suitable for a larger or more professional-feeling product. Hero Engine is very capable and probably meets most of your needs. I don't really know anything about BigWorld.
5. Allegro (C++/Pascal/...?) Library only? Import of 3D models?
Allegro is a lower-level library with similar capabilities to SDL or SFML. The built in rendering functions are for 2d games, but it can be used alongside OpenGL or Direct3D for 3d. This is almost certainly not
what you are looking for.
8. Unity (?C++?). Easy to use & import Blender models. Windows only (but could be used to write windows clients?). Source code? How easy to extend to MMORPG without source code?
only provides a Windows-based editor (for now)
You should definitely do some more research on the options you're considering, there are a number of mistakes in the information you've got thus far.
Hope that helps!
: : Corrected information about Unity, thanks to the below post from Saruman.