+1 with MartianDestiny

Having a foster animals can be very rewarding but it does take time, patience and lots of love. Older foster animals frequently have issues. Sometimes they are the sweetest thing. So its not that they all need lot more attention. It's just that some do and you don't know until you've had the foster for a day maybe even a week or two.

The other thing about foster is that if you really like the dog, you might just go ahead and adopt the dog. Many of the rescue group will allow you to adopt the animal you are fostering. I don't know of any group that will not allow you to adopt a foster animal you are taking care of.