I love the concept btw, its totally awesome, alot of people are suggesting allying in a dwarf captain with throwing axes and shield but since your choosing from the khazad dum why not go for Mardin, he's almost the same points value but has an extra might and a pleasent suprise should he defeat a troll in combat, Or better yet, take Durin, best dwarf in the game and malbeths special rule would really complement his lack of fate
Malbeth
Durin
26 warriors of arnor
13 hobbit archers
500 Points, 4 might (presuming malbeth has 1 point of might), 41 models, 13 shots, the most nigh unkillable model in the game (durin) and a 5+ save on all wounds
Or...
Malbeth
Mardin
4 vault wardens
23 warriors of arnor
12 hobbit archers
497 points, 4 might (again assuming malbeth has only one point), 45 models, 12 shots, 4 defence 9 models to clog up a tight spot on the map and nearly infinite spermen to support them should their teammate die, to make things better they have that handy 5+save on all wounds.