Definitely you want to look after the health of your babies-so like Crio said the dog needs to be healthy, vaccinated-and SAFE. Even a healthy dog is no good if they may bite your dog-or you

I know most shelters are very careful about this sort of thing-but you should be asking them questions on how they screen to ensure they are doing their part
I haven't fostered dogs before, but I'd imagine you'd want to look at the potential dog in the same way you would if you were adopting. Can this dog fit into your family? Do you have time to potty train if they aren't-or if they decide they aren't potty trained anymore? Do you have the time?
Also question the shelter on their policies. Many pay for food and treats and vet bills. So do they?
And who's expected to drive to adoption events?
Are you expected to have strangers in your home, when it's convenient for them, to meet the dog?
Or do you meet them somewhere?
Or does someone pick up the dog and take him to meet them?
Good luck

I've fostered other animals and it is rewarding, sad but awesome too

Keep us posted on your decision