The place in your block that has a crack probably will not have any effect on the internal engine. If it were mine, I would fine a machine shop in the local area and try to have them grind a little on it and then weld it up. If no one wants to do that, I would grind up the area and use liberal amounts of JB Weld (or similar). I doubt that it will cost over $100 to get this fixed unless the leak is being caused by an internal rust issue (which isn't likely in your neck of the woods).

Honestly, there is a chance that the previous owner did not notice the leak (positive thinking). This isn't the worst thing that could happen, so not a big deal.