It's nice to see the M-type axe being seriously made - the war axe is an underrated weapon compared to the sword.
As far as authenticity goes, ash would be a safe choice for the haft. The famous Mastermyr tool axes for example have ash surviving in the eyehole and it's a great wood for the purpose.
I know of only one M-type with a thickened blade being examined for it's inner structure. It has a steel bit welded between two iron plates which form the thickening on the edge. it's from Estonia and published in "The power of Iron" by Jüri Peets. It has a blade width of 19,4 cm (about 8 inches) and weighs 540g (1,2 pounds)
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A sketch of the structure on the Estonian axe