Yeah, I think the Rampage is a little complicated, mixing Vehicle rules with Monstrous Creature rules too much. Especially since it effectively makes the Engine 5 wounds, though with decreasing effectiveness as it is wounded.

I think removing Rampage, and putting it at 80pts as a 2 wound MC with Eternal Warrior should work pretty well.

In fact, it probably doesn't even need Eternal Warrior, as only stuff like Force weapons would be able to inflict Instant Death, and there has to be some models that Force Weapons can kill.