It's one of those jobs where you might consider getting another core, even one that you can repair, so that you can swap it straight in. You can then repair your own and sell it on. However, you can bypass the heater and get yours fixed and still use the car, but you'll need to seal it all up to stop the rain getting in. By passing the heater now would be a good idea while you plan your next move. The core probably vibrates a bit with the fan on and that might make the leak worse.