Why don't you look into getting the Action Pack from Microsoft. With a bit of imagination you can supposedly use the software for internal purposes virtually for free. That could save money on a second Windows 2003 Server license.
Unless you know a lot about Linux I would think that the time you're going to spend on trying to do this will almost cancel out the amount spent on an additional Windows license.
Another thing to bear in mind is that Windows 2003 RC2 supposedly solves some of the issues surrounding having to deploy branch office servers. I would have a look at that before making a hasty decision on going for broke with a Linux "domain controller", which I think is essentially what you're planning.
Sounds like an interesting problem to solve in any case - I'm ever so slightly intrigued...
What sort of connection speed between the 2 offices are you talking about?
If you manage to pull this off, then I hope you will document and post what you've done somewhere on the Net - I for one would be very interested in this kind of scenario (I'm new to LInux so don't actually know if it can be done, but the concept sounds very interesting indeed).