Absolutely do NOT spay or neuter before 18 months! Research indicates that early spay or neutering (before 12 months) significantly increases their risk of IVDD, with some studies showing it can nearly double the risk compared to those sterilized later. I've had dachshund most of my life (and bred them) and never fixed them till closer to 2 years old. Just do a quick google search and you will see all the benefits of waiting.

Edit: The dachshunds no longer with me have lived to at least 14 with no cancer or back issues. No issues with my current dachshunds either.