Our beloved Rottie was jokingly given the nickname by our family of #1 son, he really was an amazing dog, sadly missed. When our son was born he became nanny and would always sleep next to DS, if he had to go anywhere (had to being relative, he wasn't Batdog) the first thing he'd do when he got back was peer into the moses basket or try and get his head in the sling to check him out - I wonder if he thought I was up to this parenting lark! I once had to nip and get DSs book from upstairs and left the HV with DS in the basket - it turned out she had gone to pick him up and Superdog had nudged her back to the sofa and popped his head in her lap to keep her there, very gently but she clearly wasn't to handle his boy without permisson. He slept next to DS until his last day. Sorry, we're totally not over losing him evidently!
We have a Great Dane, a Great Dane x greyhound (probably, he's a bit of a mystery - but imagine that romantic interlude 
) and a Flatcoat x Smooth Collie at the moment- the only thing I would change is oldest dog's way of thanking you for a meal which is to snuggle up to you and share a great, chewy burp right in your face. Vile creature.
I would say, with any giant breed you are thinking of getting - do your homework on their hereditary conditions, see what certification or genetic testing is in place to try and erradicate them and go to a breeder that uses these methods, is open and honest with showing you the results, pedigrees, dogs and answering your questions - good breeders want the best homes for their puppies so be prepared to answer some qustions of your own. If you have decided on a breed, talk to people involved with them, get them to recommend breeders and see if names keep coming up, some may be less reticent about giving you names to avoid but expect to read between the lines. Also don't limit your travel - if you only want to buy from your area you may end up with a dog from a commercial breeder, what may be called a puppy farmer, this rarely ends well due to various factors.
Take other factors into consideration too about day to day living - grooming time, a heavy coat on a giant breed is a lot more coat to comb than a small dog, if you are going for a jowly breed will you be happy to clean and dry their lip folds everyday, do you live near enough to a GOOD vet if you have a GDV. Do you have the money for things not covered by insurance - prophylactic treatments, vaccinatons, minor ops are very much more expense for large breeds. Insurance is another one - there are varying views on insurance, it does seem to be a little unfair when it comes to giant breeds but whatever you do you must have a contingency plan - if you don't pay into insurance (which with a good policy will pay on for meds and ongoing tx after initial condition has been treated) then you really should be paying the same kind of figure into an emergency bank fund - accidents happen and if you have a giant breed it will be many thousands by the end of treatment for a broken leg for example. Chronic conditions are also going to continue to be expensive - hundreds or thousands of pounds a year for something like a giant breed with Cushings.
Lifespan - if you go to a good breeder (bearing in mind genetics is a complicated thing), grow your puppy slowly and DO NOT over exercise it while it is growing (thus reducing the most common cause of skeletal issues which is incorrect nutrition and damage to growth plates and joints from over exercise leading to growth issues, cartilage damage and arthritis etc) so no up and down stairs, no walking miles till they drop because 'they're a big dog' etc then you can reduce the chance of some factors that lead to PTS for quality of life reasons. It is a reasonable expectation to get 12 years, some lines have dogs regularly reaching 15- in breeds that go earlier due to heart complaints, this is as an issue that is being countered with good breeding practice and health monitoring. If you are told that breeders dogs usually only reach 9 or 10- ask why. However some countering of these issues is being done by investing in slightly smaller dogs and breeding on with them - if you are going for the biggest dog in the world, you may have to accept it. That said, sometimes it will happen even with everything one could possibly do having been done - that's true of all of us really though.
That sounded very ranty, it wasn't meant to - but it is sensible to do as you're doing and get as much background as you can on the good and bad.
With all that said, I love my giant breeds and would never change them, we have not had any health issues and they don't take up much space - mine sleep on memory foam beds I got handmade from a small place online, I got their collars there too, that was hundreds of pounds cheaper than getting them fitted out in other places (I saved about £300) and they even gave me 3 free cat collars and they are lush! So not everything has to be expensive! Food racks up but when you work it out what I feed vs smaller breeds it's still good value. Actually, I say they sleep on their specially bought, shaped to fit in their cubby holes beds, right now they are on my bed, with me, one DH and 3 cats - not everyone is happy with pets on the bed but by gum I'm lovely and warm!
Plus - they're just the most awesome dogs- love them...just invest in a cork for the gas (DH assures me it's the dogs).