Here's an article I would aquaint myself with - just to be armed with all the information:
What is Predatory Drift? | Go Dog Training | Blogs on DogTime.com
I will say that not all large dogs require huge amounts of exercise - research your breed. Certainly all the research I did on Westies was TRUE! They said tons of exercise and they weren't kidding. That's both bad and good.
Lucy wears out about the same time I do - Ringo - not so much. They kind of make uneven walking partners because Ringo is ready to do another mile when Lucy is ready to quit. She's small enough that I can carry her though.
Check your Homeonwers or renters insurance - like it or not - most insurance companies now restrict certain breeds and you would not have any liability protection if they are restricted. If they are a mixed breed - then that restriction is a little harder to enforce.
Good luck and make the best decision you can.