Is it safe to spay a 9-10 year old Dachshund?

I think you should evaluate all information the /new/ vet can give you on this subject, and if your dog's condition allows it without significant risks, then go ahead and do it. My poodle was never spayed, never allowed to have pups, and it was one of the reasons that she developed a uterine area cancer at only 8 years old. If we did the operation timely, we probably would never have to go through this trying time and spare her all this suffering. And 10 y.o. is not such an old age for dachshunds who have the longest life span out of all dogs (18-20 years). You really cannot decide this without a qualified vet examination. Good luck!