2 ways to look at it mate.
Im in the middle of sorting my engine out with new stem seals, Piston rings, big end shells, and a chain kit etc which has now run into about £400 + a few days of my time on and off. It should leave me with a good engine that's good for a few years.
Alternatively I could have spent not much more on a replacement engine with unkown history that could have developed or had the same problems and had to do it all again.
My point being that a new engine is a quick fix but regardless of what any seller says you have no idea what has been done to it and how well it has been done. Ultimately you have no idea how long it will be till you have the same problem.
Choice is yours.