Heart of the prototype will be a Numato Elbert V2 FPGA board. It has a Xilinx Spartan3 XC3S50A FPGA, which is somewhat unfortunate for me as a lifelong Altera guy but they all speak VHDL.

It should be large enough to handle a SDRAM core and 3 I2S interfaces. The board has 39 dedicated IOs and I am sure the 6 LEDs and 8 DIP switches can be re-purposed if necessary. The buttons might come in handy for a user interface, except the big red button of course, which will be there no matter what.  32 IOs are located on 4 PMOD connectors. the daughter card will connect through them and will mainly hold the SDRAM interface (~30GPIOs for a 64M x 8bit device). 64Mx8 is the smallest I could find with an 8bit data bus to save pins and available quantities. Also on the daughter card will be the FM radio and the line in and out interfaces. Separate control interfaces will go to an Arduino controller, could be as simple as an I2C interface. The prototype might connect to a PC for user IO, but the end goal is a small display showing the radio station and other useful data. The big red button was part of a Sparkfun dumpster dive shipment, waiting patiently for its opportunity to shine.