I am buying a new puppy at the end of summer and it's a black Miniature Schnauzer (female). But I also have a 3 - year old cat (female) and I'm worried about her going missing, not only because theres something new in the house I will have also just moved about 2 weeks earlier! Can someone please give me some advice on what I should do about the cat and the new little one?

I presume that the cat is still being kept indoors as you have recently moved. Let the cat have escape routes from the puppy either up high or to another room. After a few days the cat will get to accept the puppy. Don't let the pup mob the cat. She too has to accept that someone else shares her home.
I have brought two puppies into a house with two cats. The cats rule the roost and they all get on brilliantly and sleep together. Just take your time and give the cat the chance to come round while keeping the pup under supervision. They will very quickly sort themselves out.