After three days with the Zonbu thin client computer I stared to forget it was any different to any other desktop computer. I had a query from a "Sundowner" asking how it would go in a caravan and for banking. It should work well in a vehicle and I would be more confident of the security of a Linux computer than a Windows one for Internet banking (St George's Internet banking worked fine).
Adding a pinter will cause confusion for many people. By default documents are "printed" to PDF files and that works very well. When I plugged my old Cannon i250 ink jet printer in, the system recognized it, but then offered me an Apple printer driver. This Cannon is not in the list, and when I went to check the web what to use instead (turned out to be a bjc-7000), I somehow stopped the printer installation wit a CUPS error. The system then locked and I had to turn off the power to restart the machine. I have not been able to install a printer using the utility. While the computer locking up is annoying, my Windows XP laptop does this every few days (I then have to remove the batter to restart it) and have regular problems with printers under Windows.
The Zonbu certainly looks robust enough to survive being used in a caravan. I am not sure about RF output for a Caravan TV, you might be better off with an LCD TV which has VGA input as well.
You will need a 12 to 5 Volt adapter to run it from a car battery and will need someone to solder the unusual three pin DIN plug on the adaptor.
My limited experience of Telstra Next G is that it works fine. Just be careful of the data charges and that you get a data interface which does not need special software on the computer. I have found plugging wireless into a router and the router into the computer eliminates a lot of problems. The NetComm N3G001W 3G Wireless Router works with NextG, but is not cheap.
Security for Internet banking should be better with the Zonbu than with a Windows computer. But if your bank has any Windows/Internet Explorer specific software it may not work. I tried it with St George Bank and it worked fine.
ps: Another option would be the ASUS Eee PC diskless Linux subnotebook computer. This has a similar processor and software to the Zonbu, but includes a screen, keyboard and battery. But the last time I checked, these were not available in Australia.
The soft worked satisfactorily on a high speed Internet connection, I took it home to try my slower 256/64 kbps wireless iBurst link. As expected this worked okay; the applications and data are stored in local flash memory, along with documents. It is only when you need something from the Internet, such as the Zonbu documentation, which is on the web, you notice the slowness.