I guess I'd try to figure out the CR of the "enhanced" construct and base the cose on similar CR concstructs. You might want to change other aspects (like advancing HD) at the same time, too, so they don't end up as glass cannons. You might see if you have similar constructs available in some later monster books (or in the Creature Catalog!); I have a feeling something close enough may already exist for some of what you want.