La Grande Guerre goes for quite high prices on Ebay. I was lucky enough to buy one from the publisher in Paris, several years ago. As Calvinus, there is no way I am departing from it.
However, do keep in mind that unless you know a couple of other "fanaticals" like you are, you will hardly get any games of the boardgame going.

It is a monster! Altough a playable monster by any means.
There are modules of LGG for Cyberboard and Aide de Camp 2, both are excellently done. PBEM does work with them, but it is slow, mostly due to tactical battle rounds. Also, with PBEMing the boardgame, if you ever get to that point, aim to have only two players. Due to the nature of playing in e-mail, it is faster to have only two players, rather than four.