Well the satellite tracker arrived today (
http://r.ebay.com/u8r5K8). Popped the ALDI SIM into it, did a little bit of setup, and bingo, it worked really well.
It has a lot of different settings, but sends a text to predetermined numbers if it moves more than a certain distance or outside a predefined area. Also has the ability to drive a siren, and comes with a remote control so doubles up as an alarm as well.
Just looking on where to mount it so I can get constant 12v power, and the 3G and GPS antennas can be run, that could be a bit of a challenge. For under $45 its got to be a very good investment.