
Like said, let your breeder guide you, but I would also suggest talking to other exhibitors and breeders of border terriers. Take all advice in and see what happend. Lok at dogs you like in the show ring. I know someone who rights down a list off all the faults in her bitch, then what she wants to improve on, then lists all the faults in the dog and looks to see if there are any faults being doubled. She will do this for all possile stud dogs. Might be worth doing.
Also talk to people about lines. When you are looking at stud dpgs, get their pedigree and put it with your bitches and see what it looks like, are you doubling up on anything etc, looks at pictures, or if possible the actully dogs) of 3 generations back, maybe more. A true breeder who does the best for their breed will spend hours looking at these things, the internet can make it easy for you!
health checks your breeder should know, also if you can'tact the breed club they should tell you. Not all breeds yet need DNA as not everything has a DNA test.
Most of all good luck, enjoy you hobby of starting the shows and remember that you won't know everything but talk to lots of different people. I have seen the Border terrier year book, if you don't already have it, it looks to have some excellent information in aswell as pictures of other dogs.
Have fun.