The problem with getting a used engine is that you don't know what you're getting. Even if it shows good compression. You will also pay similar, if not more than just going the new short block route.
The part number you want is 10103AC030
. I just bought a new short block from one of our vendors here, Heuberger Motors. They are having a 25% off all parts sale, here is a link right here for the short block http://heubergersubaruparts.com/cata...5892&makeid=32
Why buy off of ebay when you can have a forum vendor and authorized Subaru dealer get you a new short block for $1645. You can also see that the part number for the short block is applicable to both the LGT and STi.
I know it's already a big expense, but some things you may also want to consider is a new 11mm STi oil pump, and a gates racing timing belt kit with water pump. You'll also want the gasket kit which is about $225 or so at Heuberger.