I want us to focus on making the games and not getting images, sprites, sound, etc. working properly. Since I'm not sure on the skills of the people joining I'm not sure what to do for the project.
Hearing that I feel that writing your own engine is not what your are looking for, but instead want to get started with the functionality of the game right away.
So you probably should search for Engines that are not too complex. Unity is probably the most known right now but I personally have never worked with it.