Don't send it back quite yet. Have a compression and leakdown test done. Get it done and show proof to the seller if the engine has problems. Antifreeze in the combustion chamber is not usually a good sign.
If the compression and leakdown tests come back alright, then I'd keep the engine. If not, give the seller the chance to make it right. After that, it's on like Donkey Kong beootch.
I don't know what to do. Should I return the engine and find another engine?