I'd recommend getting a PocketPC or PocketPC Phone Edition, since they're designed from the ground up for low-power always on use. MS gives away the C Compilier and Visual Basic for the platform. TCP/IP is built in (not to mention PPP).
You can get a low-power Bluetooth card for most PocketPCs.
I don't think there's one out for a Pocket PC (could be wrong, there's a lot of apps out there).therefore i have my webserver at home which would be able to generate the wap pages, in case there is no web-server on this specific device.
PocketPCs come with thier own batteries, and can trickle charge off the car battery so that's another check off your list. (taking a guess, I think that a good car battery should power one for a month or more)any suggestions for additional battaries?
I'm not sure if it's availble in Switzerland, but the Dell Axim has a good price, long lasting battery, Secure Digtal slot for memory and a compact flash slot (for a bluetooth card).