Synchronising data on a (power) budget

I've been trying for ages to work out how I can keep our documents synchronized on Belindas notebook and mine (and a copy at home just in case) without using something like Dropbox. The main reason I don't like using Dropbox is that if Belinda makes a change to a document it must first send the document out to the internet and then it will notify my computer and if Belindas notebook is on at the same time as mine it will sync locally.
We've been using BTSync (uses bit torrent protocol to keep your own data in sync) and it can work on windows, linux, android, apple, etc. I've looked at all sorts of notebook, NAS, etc and was going to use an old netbook computer but it suddenly hit me today.
The lowest power machine I've got is an Android phone.
Here's the solution. For internet access (and local wifi) we either use my phone tethered (Optus $2 per day prepaid which includes 500MB of data per day) or a Telstra 3G wifi hotspot which gives us 8GB per month. The Android phone running BTSync will automatically switch between whichever wifi is available and is always going to be a local connection without going near the internet.
Because this copy of the data is always available it means that if Belinda is online without me or I'm online without Belinda then it will always keep in sync with the Android meaning the 800MB of data we have wills always sync locally without going out to the internet.
Hmm, just read that back to myself and maybe I'll do a picture later. Anyway, I've proved the concept and when I go back to Canberra at the end of the month I'll bring back the Samsung Galaxy S2 with a cracked screen and have a 32GB micros SD card in it.