I had a few goals when choosing a microcontroller. I wanted a built-in Ethernet interface, USB, plenty of serial IO, ADCs and DACs. Also important was the availibility of an inexpensive development board and an unrestricted (ideally open source) build platform. From basic searching, I came up with the STM32F4 Discovery board based on the STM32F407. The '407 is available in a few friendly TQFP packages, so it will be relatively easy to create a new board using it when the time comes.
The Discovery board is really well equipped. A large majority of pins are routed out to the pin headers. For now, the main concern is being able to find enough unused pins that can map to the needed peripheral functions. Worst case scenario is that I have to start desoldering parts, but hopefully it won't come to that. The first thing to do will be to develop a daughterboard with all the needed digital functions and interfaces needed for the radio.