Check bmaparts.com first...not sure that's an OE ET and it should be.
No, the tank is gone...doesn't matter if you overfilled it or not if it's leaking...though that can cause an ET to pop. Fill to between min and max.
If you used non-OE coolant, I think OE is best.
Drain from engine block (so get new crush washer).
Fill with just distilled after replacing ET and look for leaks again. If you're good, drain from engine block again, after cooled down. If dirty, fill with distilled again and drive a little bit to op temp. Let cool, then drain again.
Then add the OE coolant straight before pre-mixing and top up to proper level with distilled. You'll have some distilled hiding in system that won't drain, and you want a full gallon of coolant in to get ratio right. For adding on subsequently, use 50/50.
So, you might want an extra gallon....not bad to keep around as you'll have some seeping and should monitor level closely anyway. It's a critical system, you know!