Great example! I use Urho3D because of its excellent document and plenty of useful examples. Most important, it is one alive project. I had used GamePlay engine before, but it is almost dead now. To keep an engine alive, the best way is using it to do some actual projects.
@SLC, it is 1.6. I think I sync'd to the head about two weeks ago.
My first guess for the Bullet compile errors is that you only grabbed the files in 63_OffroadVehicle folder? There are source/header files in Source/ThirdParty/Bullet and Source/Urho3D folders that are in the repo.
For my testing, I drop the repo into a virgin urho3d 1.6 folder and do the build to ensure all source, header, and data are copied. But on an existing built folder, I imagine the changes in the header files from the repo would require a rebuild if the dependency make files didn't pick it up automatically.
Good luck with the build and let me know how it turns out.