What is the current best practice for keeping files on machines on a local network in sync and also with Ubuntu One?

My issue: I have a local network with a large-ish amount of file data (approximately 35GB) to keep synchronised between several machines on a local network, and I also want to keep this synchronised with Ubuntu One file storage (I've purchased the 50GB account option).

My uplink to WAN is severely bandwidth constrained (up to 0.5Mbps), bandwidth capped (to 40GB/month) and intermittent (the laptop that currently holds the data will be connected and running UbuntuOne with my user account for approximately 1 hour a day) -- I estimate that with these constraints it will take approximately 23 weeks to complete my upload to Ubuntu One (if I could leave a machine permanently connected to the WAN I estimate it would still take just under 1 week to complete the upload).

I do need the machines on the local network to be consistent as soon as possible, synchronisation to UbuntuOne is less urgent (although obviously I would like to start using the storage I am paying for sooner rather than later, and it would be nice to be able to access all the data through the web interface sooner as well).

What is the current recommended best practice for such a scenario?