I own a Zangersheide stallion and have worked with many in the past, all jumpers. If you do not have extensive experience handling stallions, I would NOT advise purchasing one.
As for gelding him, if he's quiet enough (an atypical stallion) and you don't have mares in proximity, I would leave him. If it's necessary, I would take your vet's advice. I had a warmblood brought to me who had just been gelded (he was 8). He was a fantastic jumper, but he definitely still had his stallion tendencies. However, I've worked with other stallions who were relatively quiet and well mannered. It all depends how they're handled.