Insulation "R" Values
The most important thing to be aware of with insulation, other than the safety issues of urea formaldehyde and asbestos (covered elsewhere), is the R value, or the insulating factor. The higher the number, the better the insulator.
Don't be too shocked if the insulation in the 15 year old home you are considering isn't near the R value of a brand new home. While there is no question you could save on heating bills by adding more insulation, sometimes it can take many, many years to recoup the costs.
Often, gas or power companies have financial assistance programs including low interest loans or even grants available if you boost your insulation. Talk to your home inspector about any upgrade recommendations before purchasing your next home. More on insulation is available here.