Custom RPG - Which engine?

Interesting! I don't know of any "engine" off-hand that allows for this kind of customization (I'm assuming you want to do much more than what is possible in a mod).

I think Alien Swarm was released as open-source - it's a top-down third person action shooter kind of thingy. With a lot of work that might be something you can build on.

how much are you willing to pay?

at the moment, nothing!

I also seem to recall that Blender, the freeware 3d program, has some game possibilities. I haven't checked on it in about 8 years, but you may look at if someone has released things like this using blender before.

it's called "blender game"

Then I think you'd be better off modding an Elder Scroll game. Generally companies don't let you get the source code of the expensive engine they made for free.

but if I could buy, which one would you recommend Goafan77?

in fact, I am looking forward to OpenMW

Unity is free is you are self publishing or a very small company. At least to my interpretation of the EULA. (http://unity3d.com/company/legal/eula, if anyone knows legal speak input is welcome.) I'm currently building a 4x game...err prototype. It's more of a experiment in world generation and city management type game while I go to school for game design. 

http://freegamer.blogspot.co.uk/

sort of open source games news type place.. you could have a look at the various games and what not. funny thing, blender is right at the top.

Siege Engine.

I'll second the Unity engine suggestion.  Very good set of tools already with many games published.  

where is it?

there were other engines avail to indie devs like

cry engine

UDK

and unity of course.

no idea what is multithreaded though?

multithreading: exploiting the full power of a CPU.

.. think the person really meant which engine supports multithread..