If you are looking at an ST product, why not have a look at the STM32F429 discovery.
It is a 180MHz microcontroller with 8MB of SDRAM on board and a small LCD display with touch screen. A single ADC is capable of 2.4MSPS or up to 7.2MSPS in interleaved mode. It also has a built-in programmer/debugger. There are lots of examples on ST's website including a professional graphics library, an RTOS, FAT file system, JPEG decoder/encoder and many, many peripheral examples.
It is not really suitable for your own design (it may be) but it gives you a good, cheap platform to design your own from.