Debian :: Linking Remote Servers ?

We have one office with a simple samba server running on Lenny; not a PDC, just simple username/password authentication. It also acts as a DHCP server, NTP server, and DNS server. All clients are windows machines (mainly XP Pro, but also starting to get Win7 show up). For remote access, most users have chrooted WinSCP; OpenVPN was setup, but too slow for most AutoCAD uses (our primary application). It is still used by a few people, though.

I need to set up a similar system in another location, and need to provide access to both servers. People mainly access files stored in their own office, but will occasionally need to access files in the other office. In intend on having both run the same version, whether I stick with Lenny or put both on Squeeze.

I was thinking of setting up both servers, and then mounting the remote file system (the "other" server) over the internet, possibly via sshfs, but do not know if it is reliable enough. Or perhaps via OpenVPN connecting the servers, but then the overhead of the VPN may become an issue, adding lag time. The remote shares would then be accessible via the samba config.

Are there other options that may be easier to set up, more reliable, or more secure? We do not want a domain set up, just simple workgroups. Since many are using their personal laptops, joining them to the domain can cause some problems. It would also require more effort to add a new computer, and with the constant stream of outside consultants we use, that could get cumbersome.

My main issue: no budget beyond the low cost box for the second file server, so it has to be an open source solution from start to finish.

I'm not quite a newbie - I have quite a bit of experience setting up and maintaining basic Samba / DHCP / ssh servers - but not much beyond those. I certainly am open to learning new things and doing the research, but my head is swimming with all the options I have come across.