Are you getting your pup from an ethical breeder, who screened you carefully and can provide documentation of the health testing on the parents, grandparents, etc?
At minimum, a breeding Dane should be clear of hip and elbow dysplasia, cardiac clear, thyroid normal, and NO HISTORY OF SEIZURES IN THE BLOODLINE.
And absolutely most important, you need to meet both the parents (at minimum) and see for yourself that they are affable dogs with solid, nonreactive temperaments.
As a trainer, my experience with Danes is that when they are good, they are very, very good, and when they are bad, they are unspeakably dangerous.
A Dane with fear-aggression, territoriality, or dominance-aggression towards familiar people is a truly terrifying animal. These traits run in bloodlines. They seem to be most common among some of the harlequin and merle lines; I don't think this is "linked" to the color, just an artifact of idiots selecting for a lucrative flashy color and ignoring the dog's temperament.
Contact breed rescue and ask them if the breeder is known to them. There are two ways a breeder can be known to the local or national rescue. You want the good way -- the breeder who fosters for them, helps with transport, sends money, etc. The "other" kind of breeder "makes a contribution to rescue" by producing dogs that get dumped on them with expensive health problems, bad temperaments, and no safety net provided by the person who profited from their sale. Don't buy from someone who is known in this way. Just. Don't.
Some management tips: no slippery floors -- get rugs *solidly* attached everywhere. Be careful when Danes romp with each other or other large (and often more athletic) dogs -- it's common for them to get injured when playing full-contact. Like humans afflicted with giantism, they are surprisingly delicate. Use a plastic crate if possible, and monitor the pup's *tail.* Danes are one of the breeds that are prone to injure their tails, often when wagging in a wire crate, but they can whack it open on furniture and walls, too, and spray blood everywhere as they continue their happy dance. These take *forever* to heal, and sometimes won't.
Find a dog chiropractor and use him.