I think sometimes older ponies are worth their weight in gold. They have been there, done it - reliable enough for a novice to get on, thats why the price is sometimes high.
To be honest, the way things are financially, I would not be paying that sort of money. There is the strong possibility that this pony will need a lot of care as it gets older, more food, maybe vet help, stiff joints etc. You could get 10 years out of it trouble free, or there again, problems could arise sooner rather than later.
As an owner of 9 horses, and having had a few elderly ones, I am speaking from experience here.
I doubt very much a vet would vet it given its age, and you would not be able to get insurance, again due to age.
There is a lot to be said for having an older pony, and I am not wishing to put you off. But on the other hand, I have seen no end of adverts for ponies for a long term loan, young and old. Check ads in your local horse feed shop, saddlery shop etc. I am sure you can pick one up through things like that.
If you really want this pony, I would put in a very low offer, or even ask if they would loan it. You never know, if they cannot sell at that price, they may take your offer.