I did rottie rescue for years & they are great dogs & very trainable. They have a strong drive to please & if trained from the start are very managable breed.
When choosing a breeder of Rotties I would go with 1 that has many yrs breeding & countinues to improve on their dogs with imp & new bloodlines.
Health testing is a must as there are a few health issues within the breed.
The breeder should be happy to continue giving you advise & help for the lifetime of the dog. They should also be quick to advise you on correct socialization of your new pup.
I would go to a kennel that dose not breed heaps of litters every year also as their follow up care would be mim & the pups upbringing may not be of the best standard.
Of course visit any kennel in person that you are thinking of buying off.
I will probley come back to this thread & post some links for you later today.
Dogs make everyday life enjoyable...........