as for reasonable upper end expectations for the ADCs....I'd be happy if someone could pull off a 100 msps at 8 bits which would give an effective 20 MHz capability...maybe up to 50 if you push it..moving to a 12 bit, I wouldn't expect more than 50 msps....so a SERDES would be overkill but standardized (SPI can't do enough but an 8-bit parallel interface should do just fine)