it's the Esenthel engine I'm testing beside using and loving 3DGS: http://esenthel.com/?id=news terrain streaming is a great feature of it!
its editors are very cool, ensuring fast and easy workflow, but you really need experience in C++. it's mainly suited for MMO RPG games as I see. so if its features are okay for you, you might do the job with little programming based on example projects. it has a navmesh pathfinder without dynamic avoidance, and there is an example in the tutorials of a grid/tile based pathfinder too. moreover you can extend its grid system due to your own needs (see grid.h in header browser).