Well, that's your first mistake. You don't own them, they own you!! There are several important factors to consider prior to purchasing either a Great Dane puppy or an older Great Dane.
1. Size
2. Life Span
3. Health Issues
4. Nutrition
5. Costs Associated with a large breed
SIZE
If you want a Great Dane just because it's "SO BIG", you aren't ready for a Great Dane. If you have small children or elderly people in your home you need to think twice. Great Danes are very gentle, but they can easily knock over small children and older family members by accident.
LIFE SPAN