98 Explorer catalytic conv

Just bought my son 98 Explorer XLT. Seller said check engine light had just come on. Took it to mechanic for computer read out and he says the catalytic converter needs replacing. Ford Parts quotes $702.
Is it OK to replace this with a junkyard converter to save money? Can this be done myself in my driveway or is it necessary to pay a mechanic??

Thanks for any suggestions.

Never seen a cat conv go out, rust out, yes. Reset the check eng light and find you a different mechanic