Take a look at the Udoo Bolt or their other products. Basically that's an x86 SBC with proper RAM slots.
I forgot about dual Ethernet requirement. You could possibly go for a USB 3.0 adapter if it's acceptable.
A quick search shows you might have some luck with Beaglebone or Beagleboard with regards to emulation. OTOH those boards are cheaper then most JTAG debuggers.
I know of three companies manufacturing Cortex-A processors which have openly downloadable documentation:
Texas Instruments (theirs are in the Beaglebone)