Were you aware that there are different types within the GSD "umbrella"?
There are the Show lines - mainly black & gold, some sable (though different from W/L sable. They mainly have sloping backs. Bred in this country, purely for their looks. Nerves can be an issue.
Interestingly Germany requires S/L GSDs breeders to ensure that their breeding dogs also have working qualifications.
The Pet lines - mainly long haired and multi coloured, including the whites. Usually straight backed. Some lines have bad hip dysplasia & epilepsy. Bred for their coats/colour for the pet market.
Then the Working lines - mainly sable, bi-colour, black or black & tan. Within these are the "sports" lines & the working/police lines (which I have). Bred purely for their working ability, so must form strong bonds with their handler and want nothing other than to please their handler.
The ones you've met through work are more than likely the working line ones &, in my experience, nothing at all temperament wise like the long hair pet lines that you are thinking of getting. Remember the police need entirely different qualities - brave/loyal/protective/intelligent/integrity/good health etc - from a dog , so aren't so bothered about coat length/colour/looks - just that it's got the right temperament make up to get the bad guy.
Whereas the pet line breeder will be breeding for the latest "in" colour - which is not in the breed standard - or coat length & not necessarily for the correct GSD temperament.
Of course there will always be exceptions in and out of the specific lines, but generally this is what I've found over the years.
I think that you should have a long hard think exactly what you want from your GSD. and if you have still have your heart set on a long haired one, then at least make sure you get it from a breeder that doesn't specifically breed for long coats. L/C occasionally occur in normal coated litters. From what I gather, the L/C pups used(decades ago) to be culled as they weren't deemed suitable for work as they didn't have an undercoat.
Have you considered one of the police "rejects"? You're in an ideal situation for one, plus it will have been house/obedience trained, out of the puppy mouthing stage etc. GSD puppies are really full on with their mouthing & playing, so this will need very careful supervision with your old Cav & your daughter as they can unintentionally hurt when playing when they're pups/juveniles, an older dog should have outgrown this .
I've found the females much more "serious" & not so full on as the males (mine have been slow to mature "goof balls"). Though, if your Cav is a female, you're probably better off with a male.