first off, I would like to commend you for thinking this all the way through. So many first time horse owners dive in head first, without thinking about long term care, or whether or not they can even afford it.
You're definitely on the right track. A been-there-done-that horse is always the best choice for first time owners, inexperienced riders, or people with little confidence around horses. However, many people tend to lable older horses as a 'waste' or as 'almost used up' and figure they shouldn't bother- even when its completely the opposite.
There are many different views on when to consider a horse 'old.' Many people will set a certain age such as 25, and say 'alright, this horse is now 25 so its old and shouldnt be used anymore.' this, atleast in my opinion, is wrong. The average life expectancy of a horse is 25-30 years, but with all of the healthcare available now days, many horses live well into their thirties.
Even so, older horses are in every way capable of working as long as their physical condition is good. Good joint care and conditioning can keep an average horse working into their twenties, and even thirties- comfortably.
If you bought a 12-15 year old horse with a good foundation already put on them, and made sure to provide good vetenary care for it (teeth floating, vaccines, shodding, etc) you could get 10+ years of intense riding out of it, as well as 3-5 years of moderate work such as walking, trotting, occasional canters, and trailriding. In fact, riding senior horses is often thought to be benificial to even older horses with minor artheritis, because it keeps the joints loose and mobile, and the muscles strong.
I personally think a horse should be ridden until it is no longer able to comfortably hold a rider due to joint, heart, or nerve problems, etc. We have a 49 year old quarterhorse mare who is still ridden twice a week at a walk and trot, and she's in fantastic condition BECAUSE we ride her. We believe that working a horse can actually prolong their life, as long as it is done carefully and in moderation. You just have to remember that the older a horse gets, the longer it takes to get it into condition, and the quicker it can fall out of condition.
If your horse does end up having to be retired because of physical issues (having a vet check the animal for any joint problems or prospective problems before buying can eliminate or atleast greatly decrease the chance of getting a faulty horse) there isn't too much you can do, unfortunately. You can choose to sell it as a pasture puff to someone who needs a companion animal, or you can choose to keep it. This all depends on you personally.
All in all, I think if you choose your horse carefully, a 10-15 year old would be a very wise choice for you. Buying an animal with good conformation, no history of health problems, etc can help a lot, so I'd seek help from someone experienced when buying your horse.
I wish you luck!
Everyone in your life is meant to
be in your journey, but not all of
them are meant to stay till the end.