I started off playing on impossible, but this turned out to be a bad idea Stuck it on hard for now. You do seem to get a lot of ammo, but I'm wondering if that is connected to all the weapons using the same ammo pool. It's not like the old games where you could switch if you ran out.
And like Private Floyd says, the Evangeliser is ridiculous. It may actually be the perfect Dead Space weapon - low ammo consumption, high precision, long range, rapid fire with a devasting close range alternate fire. It doesn't look (or sound) as cool as the Negotiator though.