I've never heard of anyone having wp problems on the zetec. What it looks like from the outside isn't going to really tell you much of anything about the real condition of it anyhow. What really matters is the impeller and even if you could see that, it might look fine right up until it blew. Fatigue cracks are a biatch like that.
If I had a Zetec, I wouldn't worry about it and save the money for the turbo that you're getting.
