That's relatively low mileage, and scarcely even broken in yet.
Turbo concerns are unjustified at that mileage. I know of a modded 02 WRX still going strong at nearly 200k miles on the stock turbo.
There are many ways to tell if the car was previously modded and put back to stock, and/or if it was abused. For example, all the major component bolts are marked at the factory so you can tell if a particular part has been removed and replaced because the markings won't line up.
Don't believe all the FUD though. Subaru says that the average age of STI buyers is over 40. Although a number of STI's do fall into abusive hands, it's the WRX's, with their significantly lower price tag, that you need to be more worried about having been punked by some kid.
Another thing that you could is take a pic of the car and then post it over at the
Southern CA club on Nasioc.com. Only about 3k STI's are sold in the US each year. So if this one had been modded and raced, it's pretty likely that someone in that area would know about it.
edit: I have never understood the squeaks and rattles complaints. That's what all performance cars do. If you're looking for quiet and comfy you shouldn't even be considering an STI.