Three-year-old wagon for $15k? This is a tough one, as the only offered recently in your price range is the Volkswagen Jetta SportWagen. With a diesel the VW is very efficient, and with any engine it handles pretty well. Reliability is so-so.
The new 2015+ Golf SportWagen will likely be more reliable, and handles better, but unless dealers are discounting them massively (which is a possibility) you'll have trouble finding one for $15k. Worth checking.
If you can spend a little more and/or go a little older, you might be able to find an Acura TSX wagon or a Subaru Outback in your price range. The Subaru can hold the most of the three, and the Acura the least.