The SideKick has been popular with the hearing impaired. T-Mobile carries it and they have a variety of text messaging plans, and it has a QWERTY keyboard, camera and Bluetooth, but no GPS. And as far as I know, they'll still set up SideKicks with text and data but no voice plan (most other phones require a voice plan).
Another possibility is the BlackBerry 8800, which has a keyboard, GPS, Bluetooth (but no camera) or the BlackBerry Curve which has a camera but no GPS.
Sorry I'm not familiar with tty functionality on mobile phones, so I can't address that point.
-------------------- Lisa Gade Editor in Chief, MobileTechReview