Yes, I was already persuaded by a friend to create the game first, the GUI later. I was just trying to think ahead, but I got enough obstacles as it is in this starting phase, like choosing to implement interfaces or using abstracts (if they're of any relevance at all), or which class should control X variables, etc...
On the other hand, I was taught (a long time ago) to draw these kinds of projects and being also able to draw the GUI that would go with them, which is why I feel a bit frustrated.
Following your advices, I can try to create the game first and just check a few important functions on the command prompt. Let's see how it goes.
One thing I didn't mention : One of the main objectives is making this playable over the internet. Should this be taken in account at this point, or later?