If you are more of a programmer, you could probably do well just grabbing some code from the various projects on websites like http://www.codeplex.com. Alot of people have very expansive engines, and provide the code for free with no royalties, etc. You could probably take one of those projects and build upon it, or do some work to fit a few various projects into one.
Or, if you are willing to pay but want a more stable solution, you could, like the guy above me said, look into GarageGames' TorqueX engine.
