I'm happy that there are other forum members interested in micro controllers. I had expected there wouldn't be much interest around this thread!
Some of the modules in the 900mhz band claim miles of range. I've never actually experimented with RF stuff, but if I was in the suburbs with less buildings this would be a pretty sweet option to avoid the cell bill. Now you've got me thinking about doing something hybrid so I can still talk to the car when I'm in a no service area...
sound system is cranking, but with the hall sensors, that is too damn expensive.
wifi etc. Also, you could get really crazy and get an AT&T (they support GSM and LTE as far as I know) sim card and then implement something to switch the sim between the GSM modem and a USB lte modem for the computer I think sim card A/B switches actually do exist. There are also USB host shields for the arduino which theoretically let you talk to USB modems. Unfortunately I had a pretty rough time getting that to work and opted for the most direct/reliable solution - serial.