Hi GGb - welcome to the forum. Your question is quite sound, and is a common one for newbies interested in learning more.
The color pattern you are looking for is called a Blanket Back - the black of the black and tan covers most of the back going down the legs as you described, instead of just being the traditional saddle pattern.
This pattern is most often found in working lines, or American Show lines, and not very likely to be found in German Show lines.
If you are not familiar with the different lines of GSDs, this is a good article that describes the differences, and goes over how the different lines came about
(Types of German Shepherds, by Wildhaus Kennels )
It is hard to tell how the puppies will end up. Black and Tan pups usually are born almost solid black with some tan on them, then lighten up as they get older. The first thing for you is to find out what lines of GSDs the breeder you talked to is breeding. If German Show lines, the probability of having a blanket back pup is extremely low, almost nada.
So that leaves American Show lines, and working lines - two very different type of dogs in looks and temperament, though a great pet and companion can be found in both lines. You may want to study more about the two lines, and go visit breeders and get a feel of the dogs, and decide which type you like best. Then to get a blanket back, see what the parents look like. Two saddle backs are not very likely to produce a blanket back, but two blanket backs, or dogs coming from lines that have a number of blanket backs in their pedigree will have a better chance of producing a blanket back.