First of all, you need to be with a professional that will gives you his advices.
What you can look at is his origins (if he comes from good jumper family, he will probably have the same qualities and personality, it depends obviously on your budget).
Then he has to have a good propulsion, powerful hindquarters, strong hock, the wither has to be higher than the croup, the shoulder should be oblique.
Concerning the gait; he has to be supple, balanced, and light. He needs to be reactive to your aids and have blood for competitions.
Concerning the jump quality, good jumpers are compared to felines (they are very round on the circle and very supple).
The age of the horse depends on your level and goals(a young horse is usually more for an experimented rider while an old horse; above 10yo is for beginners in horse riding or competitions).
You also have to be careful with the temper of the horse (if it's a stallion, he'll be hot and difficult to manage, you need to be a dominant rider while a gelding horse will be easier to live with.)
I hope i could help you, don't forget to make a vet visit for him to be sure there is no health problem before you buy him :)