I am going to assure this is not the white limited roadster I keep seeing in Reykjavík but a $6k CF would indicate some problems could be expected.
More input is required than the mechanic says....
Did the car overheat. How did you determine something was wrong. How far did you drive it. "Shot" is a term more suited to bullet wounds than engine parts. Terms like cracked, broken, scored cary more weight.

My thought would be to look for justification NOT to replace the head gasket(s) but I have know idea of this cost. If it's more than 700 I think I would go with the used engine.

$2k out the door for a 40k mile engine seems like a good price. Rebuilding the existing engine would be a no no.