Been down this road in 2016.
My motor blew in a big way leaving windows in the top, side and bottom of the block...no core exchange here.
I decided to go with a 'new' OEM crate motor from BMW = $20k. I had all hoses, motor and trans mounts, belts, injectors, plugs, fluids, filters, trans gaskets, and at least a couple of other items in the engine bay that are escaping me replaced. All in $34k invoice out the door (I used my local BMW dealer).
In hindsight, for another $5k I could have purchased a built motor from someone like
Carbahn Autoworks.
Anyway you slice it, this sucks big time.
My recommendation is to buy a quality built motor. You are upside down on the money already, so buy a high quality motor and enjoy for the car for the next decade.