The Arduino Due is a Cortex-M3-based board.
The Aruino Due is definitely one good option – but there are many others
good low cost options too that have free tools. For example there are
the STM32 discovery boards, MSP432
boards, Silicon Labs starter
If you go for the Due hardware then download and install Atmel Studio
too and you will probably find it is very easy to use that to program
and debug the Due board. I’m not sure if you will also need a separate
debug interface, probably not, but best to check.
If you don’t mind using a simulator instead of real hardware, then you
could start with a FreeRTOS project that runs in the Keil uVision
simulator – there are some links on the FreeRTOS.org site somewhere. If
you go that route then you will be simulating real Cortex-M hardware –
another alternative is to use a simple demo in the FreeRTOS
, but that is a separate port rather than simulating