Both sound like a fair price/investment. I would lean more towards the 8 year old because of the age but since you do not really plan to jump. The 18 year old, as long as it is in good health, is a good option. Especially if you are looking for a confidence builder. I have a 27 year old QH that can run circles around my youngsters.
You many end up buying a more expensive feed and having teeth floats more often. You may consider a joint supplement and heavier blankets in the winter but the older ones can really be a joy.