- display many objects at once (LOD, occlusion culling)
almost all engines (free or not) support this. "Many" however, is a loose term.
again, I think you'd be hard-pressed to find an engine that didn't do this
What exactly do you mean by "up to date"? Are you looking for the quality of crysis, unreal, doom 3, hl2? I say this because I love all of the great websites advertising a "next gen" engine with hl1 graphics. Also, keep in mind that a big part of the visuals looking good is realted to the quality of artwork that goes into the game.
- "game"-logic is already present in c# code
the C# part I have no clue about, "game logic" being pre-written however seems like a contradiction, because if that was the case you would be looking for a game engine, not a rendering engine. Of course, by "game logic", I assume you mean things like AI and object tracking/world managers ect.
The biggest request is the reusability of the existing code.
You mean you want to use your code you've already written in your own engine in the new rendering engine? So long as your rendering code has stayed fairly seperated from your game code, this should not be a problem.
It is the goal to achieve better visuals (detailed objects) and higher performance displaying lots of 3D elements (up to 200.000, many might be occluded).
No... just... no
You will not find an engine anywhere which can render 200,000 seperate objects, much less an indie one. Can the engine keep track of 200,000 objects? Its possible but it would have to stream from the hard-disk. For one thing, just invision the CPU speed implications of that. Do you have any idea how long it takes for the computer to just do:
for (i = 0; i \< 200000, i++)
Now, it is possible to render 200,000 visually seperated (but not logically seperated) objects (ie pebbles on the ground)? Yes, but they need to either be part of a texture or built into a larger static mesh, and that is more an art challenge than anything. On top of all this, I can't think of any game concept where renderig 200,000 objects would be neccessary.
As for some recommendations:
Irrlicht (not sure how "up to date" it is but its REALLY easy to use, and modify I believe)
Open Scene Graph (toolkit that has a lot of the culling and LOD techniques you described)
EDIT: I've been looking into irrlicht some more recently. Its not top-quality graphics, but for what its capable of, its really, REALLY easy... like... its rediculous. Getting an animated character on screen is like 20 lines of code.