I had a similar setup to this awhile back. You have to port the number to your VoIP provider of choice and then decide on what client you are going to run (no need for SIM card). I was wanting voice service and only needed limited SMS, so I went with linphone (and played around with zoiper too). If you are needing good SMS support, then JMP is probably the best. It supports both SMS and MMS. You won't get E911 access I believe, but as data only its a good solution.
Free wifi is all over the place and if you wire up a mobile hotspot in your car (yes it somewhat defeats the purpose), you can get some pretty decent coverage.