About the life span, personally I am a bit concerned about taking on animal with 7-8 years life span.
Of course, you never know how long you're going to be lucky enough to have anyone around. But it does seem like a very short time, and it's something I have talked to OH about quite a bit. Especially as I wouldn't mind taking on an adult dog - rescue/ex-breeding, rather than a puppy. But we'll see.
As for space - we'll we won't be getting any dogs until we've moved to Sweden, where we've bought an old farm. It's got an enormous barn and 50 acres of land (fields and forest), so I'm thinking we've got enough space.
We're clearly going to have to change cars, though... We have a Mini at the moment.