I had been looking for a spare engine for my car for the past couple of months. In my 'research' I found out that the vast majority of these engines are Nissan Recall engines, that were dumped by Nissan. There is something wrong with some of these engine, that is the reason why they are offered at much lower prices. After learning that, I didn't feel comfortable too comfortable buying one these engines.
My other option was to locate an engine out of a wrecked GTR. I entertained that idea for a while before realizing that buying an engine out a car that most probably had a violent collision might not be a good idea, because there is no way to tell the engine's condition without tearing it apart. So I didn't want to go this route either.
My final option was to source out a brand new block and heads from Nissan and build it. To my surprise, the costs were actually really competitive. So I went this route.
I would highly advise anyone to do their research before buying these used engines.