Well, Just be sure to keep as much as possible "resource"-based... If the project fails you have some great stuff to give away or give other people a change to continue it (like M.I.A. I hate it when content that people worked hard on get lost)

Also, people can continue to fight about what engine/language it use, but that shouldn't really matter. First concentrate on a working prototype, to see if the game is any fun at all (GM is fine for that). If the project is fun at that point, more serious programmers will think about joining the project. A custom engine can enable you to do cool stuff, but take a lot of time, and if it's not sure if the project is going anywhere.. why waste your time ?