I'd start with buying 1 mega as every faction can include one (and only one) as an ally.
Then I'd either:
•Build it as the one that allies with my force - if I only had 1 Sigmar army/played only one overall alliance.
•If I played forces from several alliances (and I do) I'd make it as an amalgamation of all three sets of parts so I could claim he's whatevers needed that day.
Make him a little three sided, magnetized, name plate for his base as the allies are all named.
And this is indeed my plan for when I someday pick up a 2nd
GW Mega.