First off, I'd like to say I'm sorry to hear about the loss of your Harley, and congrats on deciding to bring home a new family member.

As a past Great Dane owner, I feel that there isn't really any other breed that even comes close (well except Rotties
). I can see both positive and negative points about your situation with getting a Dane.
Pro:
Danes can and do make great city companions. They don't require a lot of exercise, but they still occasionally do need to have a good gallop, to burn off some steam. Quite often their a great choice for those living in town homes or condos.
Danes are relatively calm dogs, meaning that they don't have the excitability level as say a Border Collie; making them ideal for city dwelling.
When raised with them, Danes can be wonderful around cats and other dogs.
Danes aren't major barkers (
Usually)
Con:
Danes are not nimble footed dogs! They are not what I would call a sure footed breed. They can be clumsy around stairs, and in tight spaces during their growth periods can be a recipe for disaster.
You mentioned that you work 8-5 and are requiring a dog that is capable of being left alone all day. Danes are Velcro dogs; they become deeply bonded with you, and like to be near their families, the breed is just not very independent like some other breeds. So, unless you will be getting a dog walker to come daily, then this may be an issue. The are some Danes who are able to tolerate long spans of alone time, but others can't take it, and become bored or destructive.
Seeing as you may be considering a rescue, a word of note: A lot of Danes in rescue suffer from separation anxiety, which in itself isn't bad, but can become extremely difficult to correct of this behavior. Not all have it, but be sure to ask for as much info as possible from the rescue about the potential dog. Also too, Danes suffer from a long list of health issues, which can run up quite the medical bill. All dogs are costly, but most meds and equipment with Danes is charged by weight.
A really resourceful website is: http://www.danesonline.com/ You can find rescues in your area, as well as breeders and products tailored to Dane ownership.
Great Dane Rescue Inc.: http://www.greatdanerescueinc.com/ A network of Dane enthusiasts who provide rescue in both Canada and U.S.
Another rescue website is Harlequin Haven. They don't rescue outside of Ohio, but provide an extensive list of Dane rescue contacts: http://www.hhdane.com/links/rgroups.htm
The parent club website: http://www.gdcc.ca/ Canada's http://www.gdca.org/ America's
Sorry that I have listed more con's than pro's, but I truly feel that Danes have so many pluses, that one has to face the reality about their con's. They aren't long lived, they drool (which can bother some people), they blow serious coat twice a year, but sprinkle year round, but they also steal your heart. They are gentle giants, and are special dogs for special people.
Good Luck on your research! Keep us posted on your search!
P.s. Have you considered a Bullmastiff? They're a large breed, low energy, short haired and are gentle with children and family. Great site to check out is: http://bullmastiffinfo.org/
Ashley