the answer to my question is yes, i can have an M.2 boot and 2x 2TB sata 3.5 in raid mirror.  i just did it.  looks like i will have to buy a different m.2 for the 1TB part.  I ordered an Intel 670p 1TB  I will report back once it is in and working.  Dell's answer was no they wont configure it that way from the factory.  okay, but can i do it. yes i can.
i just bought a precision 3650.  it had a factory m.2 that was booting in raid on in bios.  i bought a new 1TB msi spatium m.2 and it would not boot unless i switched to achi.  is there something i can do to run a boot 1TB m.2 and 2x 2TB sata 3.5" in raid mirror.  i current am doing this on a gaming 5680. and it works fine.  do i need another brand of m.2  i dont see why the current one will boot in achi and not raid on. when the other one does.
precision 3650 m.2 and sata raid 1