I have to agree with what is said, please reconsider the thought of "having just one litter". The shelters and rescue organizations need fosters for the many pregnant cats that come in, and would love to have someone like you to help care for the Mom and her babies.
There are health reasons why spay and neutering is so important. If fostering kittens is something you enjoy, you could continue. If breeding purebred cats is something you have thought about, find a breeder who will help and mentor you. You will find breeders who have bred 5,10, 20 years who are still learning.
You also need to think about the expense of having just one litter. Will you need to buy stud service, or do you have the male now? What if she has difficulty during delivery, will you know what to do in time? If she needs a c-section, can you afford that? If the kittens survive after the c-section, do you know have the time to feed the babies every few hours, even if your tired. If you work outside the home, can you call in to continue caring for the newborn kittens?
Please, read this post as though we are friends, and I am just warning you of what is involved. It isn't meant to offend you. Please just think about the benefits of spaying, and the complications involved in breeding for "just one litter".