30 year old furnace

[Mary R.]

The gas furnace in a house I'm buying is 30 years old. From what i've been told by neighbors, and from what I see of the building--haven't had it inspected yet--the owner had it 26 of those years and took good care of the house.
Should I expect to replace it soon? Will a new model make a difference in the gas bill? Since this has been (probably) well taken care of, might I expect low maintenance/parts replacement and gas bills if I keep this one for a few more years?
Would you just go ahead and replace it, or pay for an inspection first and go from there?