At this price I would definitely ask for a thorough vet check. It says in the ad "She has now come back into full work" - what does that mean? Was she injured or something and not in full work for that reason?
Brutally said - it's with older horses like with old cars - they may keep on going forever, but if they break down it's usually a pretty cost intensive "repair". There are many horses around that are still going strong with 20 and older, but there are even more that start to have problems with 15 and younger, especially when they were used in competition or similar hard work (ranch horses, riding school horses, ...). Keep in mind: you are most likely looking at a higher vet and/or feed bill and this should be reflected in the purchase price.
Good luck and have fun horse shopping!