Ouch, if it's any consolation (and i doubt it is) as far as problems go on those engines and boxes. that one is almost unheard of kinda rare for them BOTH to go and normally when one does, because it's seeping out one side you get plenty warning with slimey puddles stinking of cat wee (hate gear oil!).. Both going then yeah, i can imagine when moving oil could be lost pretty quickly and not really noticeable until the noise started.
My point being because it's not THAT common, a second hand box from a reputable breaker shouldn't be too big a risk IMO.
I'm guessing it'll be the diff/cup seals though? the shafts on that box should be dry, not wet so if a shaft boot/seal went all you'd lose is grease from the gaitor/damage a CV. The actual box would be fine. If the oil comes out from behind the cups seals then yeah, that's when the internals run dry and it goes kaput.