Be sure to ask the seller for all information on any leaks they have had, whether poly piping related or not.
If the seller has had several leaks, you may be able to file a claim for re-plumbing, although the time frame for filing a claim may have expired. Go to this link http://www.polybutylene.com for more information. Also try this link http://www.masterplumbers.com/plumbing/wwwboard/Forum1/HTML/002003.html for more sources of information.
Be sure that no poly piping is exposed to sunlight as it is not sunlight resistant. This includes exposed poly at the water heater inside the house, especially in garages where sunlight fills the garage when the vehicle door is open. Also on the exterior where the water heater T&P discharge line is exposed.