Based on what you're saying regarding file sizes and number of machines,you might consider setting up a secure FTP server. I'm a big fan of the
CoreFTP product line (
www.coreftp.com ). It's a fast SFTP solution that works transparently behind most firewalls. I like that because I'm way too impatient to be screwing around with the port-forward settings on a router at this stage of my life. A free client is available for both personal
and business use. (Note: the CoreFTP server software
isn't free. But it only costs $50 US so it's still affordable.)
But if you really are serious about going with a P2P sharing solution, the only one I would recommend is
Freenet. Freenet allows you to set up your own private
web-of-trust for file sharing and other web services. It was designed from the ground up for security, so if you run it in what is called the "private" mode (i.e. as a
darknet), only the machines you designate can access what amounts to your own private internet.
My company participates in one such file sharing darknet. It was set up by a group of us who do IT consulting work. We use Freenet in conjunction with the
Frost client add-on.
Frost works much like a Usenet group. It allows us to securely create message threads, and transfer files without worrying about authentication or file integrity. This is important since most of what we transfer back and forth are custom server patches, GPOs, and scripts that get installed on client machines.
In addition to classic filesharing, Freenet can also provide private torrents via an add-on called FreeMulET. Can't vouch for how well FreeMulET works however, since we don't use it.
Freenet is a bit on the technical side since it's yet another ongoing FOSS project. But it's not especially hard to set up or use provided you have at least some technical background. (IMHO: TightVNC is a lot harder to set up if you want something to compare it to.)
Clients are available for Windows, Macintosh, and Linux, which is always a plus in our book!
We've been happy with it.

Info and links:
Freenet homepage:
www.freenetproject.orgFrost homepage:
www.jtcfrost.sourceforge.net 