Howdy, a few weeks back I also picked up the Horrek's Dreadlance box! My post and questions I asked should be on this last page since you have some similar questions to me.
Stalkers have a better defined role in the army and more potential synergy than the Guard, IMO. With the former's ability to change stances towards one that gives +1 rend, +1 damage on all attacks, Stalkers are a premiere melee damage dealer for the army, on average doing 10.41 damage on a 4+ save. Their command ability let's them reroll run and charge rolls, so if you run them in Stalliarch Lords (gives you run and charge), these guys can mulch in combat. Guard on the other hand have a command ability that buffs the damage output of their shields and are a 12 wound meat shield for your foot Mortisans at 190 points.
On the point of the Mortarchs, Arkhan the Black IMO is pretty viable if you don't want to take Nagash. He's a hero monster with a good move, bonuses to cast, triple cast wizard, access to all spells in the OBR lore. He can be shot down by a volley of Lumineth Sentinels but honestly all non-god characters get shot down right now.