For the reference broad, I got Open1768 from WaveShare. It cost me around $90 USD. NXP LPC1768 is the core of this board.
Maximum clock frequency : 100Mhz
512KB Flash memory
It lead out all the pin which is convenience for user to access those function. It provide lot of interface, such as SPI, I2C, I2S, UART, PS2 and CAN. Also, it has GPIO, ADC and some of GPIO grouped to from a LCD interface. Although, the LCD interface is non-standard, it works quite good with LCD shield which WaveShare provided.
The crsytal on board is 12Mhz. With PLL on chips, this core clock can be set to the maximum of 100Mhz. The JTAG/SWD port is quite useful for debug.