For the rarity, I don't know : I'm more a repairman than an Omega collector. Maybe someone else can chime in. My generic advice would be buying this for gold scrap value plus something if the case/dial is attractive to you. You could always search for a movement if this one is really totalled, at worst.
If you have access to a watchmaker, maybe ask him to remove the movement, weight the gold so that the seller is not shorted on the gold value, at least. This is what I do when I buy gold watches. Then I add some value, depending on the attractivity of the watch and the work to be done on it.
I can't really tell you anything on the repair without having the watch in my hands: it there is rust inside, it is not a regular service, more of an open ended bill. In my area, cost of a service + generic crystal + omega crown (if found) could cost you 180-300eur. If you have to find parts, a donor movement, or perform tricky repairs it can add up on top.
If this watch really speaks to you, test the waters first on the potential repair bill (search for a movement on ebay and see for how much they go for, go talk to a watchmaker in your area, ...).
