Well, after months of just sharing the internet connection using the PC that the modem is plugged into, I finally bit the bullet and ordered a Raspberry Pi to try this out. I got an externally powered USB hub, so hopefully that will take care of the USB power issues. The modem draws 650ma max while transmitting, and I'm sure that the Rpi can't supply that without a
Raspberry Pi Power Supply.
I may order one of those in the future simply to tidy up the installation, but for now, the USB hub was only $15.
As a status update, using the 3G modem with external antenna has worked so well, I've now got 4 of them setup here, and I was just contacted by someone on behalf of the village school to try to get a setup there. It's the only school in our district without internet. If the Rpi setup works nicely, I'll eventually do another one for the school.