I'm considering running a game with a variety of house rules some of which would significantly effect necropolitans. As in Pathfinder, all undead use charisma instead of constitution to determine bonus HP, fortitude saves, and every other thing normally based on constitution. Also as in pathfinder, undead get d8 racial HD and undead templates do not change class HD. Using a variant from unearthed arcana, each time a creature takes damage, an amount of damage equal to its armor bonus is converted to nonlethal damage.
These rules make necropolitan a good choice for a charisma-focused character by increasing single attribute dependence. They also make necropolitan a good choice for a heavily armored character, by effectively giving DR 15/- (assuming +5 Mechanus gear) on top of any other DR.
So, if I were to give necropolitans a level adjustment, what would be fair? Assume I get rid of the level loss normally required.