I agree with your local shop that if you can run a full tank through it (at proper operating temperature) and it idles properly, doesn't bog when moving through it's rpm range, and the price is right. Unless you know the history of the engine, whether it has been run hot, how much fuel has been through it, etc., the price of the used engine plus the cost of a new piston, sleeve, and connecting rod should be no more than the cost of the new engine.

Used price + rebuild parts <= new engine

Lots of luck