Be wary of publishers. They are like hawks after delicious eggs. If this team were to be picked up by Atari, for example, a contract would be signed that would likely go like this: You have one year to complete the project. We get 60% of the profits (or more) and you'll be funded with $400,000. You will be required to meet milestones each month (likely, per week). If you don't meet them to our standards, we'll rip the rug out from under you at any time and hand it over to another team to finish. Have a good day.
However, the more complete this game is before officially showing it off, the better. If you can create a working, fun demo to show off to Atari it may pay off. Or they may wave a legal wand at you and try to claim it is their property, I don't know. If you were to switch gears and make it a non X-com game, you could shop the demo out to any publisher and see who bites.
Since it's running off the X-Com franchise, your first and only customer needs to be Atari. If they aren't interested, then you'll have to switch it over to an original IP so you can do what you want with it (make it free or publish it). It'd be great if Atari would be interested but I'm really unclear about fans creating games with an IP that is already copyrighted, and then shopping it to the owner of the IP. Atari may look at it and say, "Hey, looks great and they've already done a lot of the work, let's give them a contract." The thing with publishers is, if there is money to be made, they'll pay attention.