As has been mentioned a used engine is a terrible idea. You're married to it and then ANYTHING that goes wrong in the future will be blamed on you. This is assuming it runs correctly once the swap is completed. These things aren't always plug and play like the text book says they should be.
New rattles, inoperable lights, codes of ANY kind, stereo problems, parasitic draws, grinding, whirring, pulling, wandering.....it'll all be something you did during the engine swap
It's either stand your ground or buy the car and crush it so it can't haunt you.