Comment Ogre (Score 1) 237
Ogre is a 3D rendering engine with a very large community based around it. We used it for a proof of concept for a real-time simulator, and there were few limitations we faced in using it. It might not be as clean as unity, but it has more flexibility in licensing, as long as you don't might copy left, which in your situation you shouldn't. Actually, I just looked and it seems they made the switch to the MIT licence.
The proof of concept was actually a major improvement over the production simulator, but of course business politics always wins in the end.