Marth would most likely be Mercenary and Hero due to the fact that that class tree is pretty balanced like the base Lord class. Also because Marth needs a Shield.
btw, funny thing about Marth's merc model: Since his head is basically short-haired Lucina's head, which is a female head, when paired with the male version of the merc's body his head will look too small and it'll also be slightly disconnected from the body. Just a funny and kind of disturbing little glitch.