It's great that you want to rescue! I'd definitely recommend doing that, if at all possible. Check out
Petfinder.com: Adopt a pet and help an animal shelter rescue a puppy or kitten.! They always have TONS of dogs that need rescuing. Also, look into the Yorkshire Terrier National Rescue:
Yorkshire Terrier National Rescue Inc. Information
As far as your questions go:
Can yorkies do stairs? Yes! Franklin runs up our very steep stairs all the time. The only thing is, he can't come down.

So we have to gate off the stairs. It probably just depends on how steep your stairs are and how big your dog is.
Do most of you use puppy pads/puppy litter box or do you potty train outside? Everybody here is different! Franklin uses potty pads but there are many Yorkies on here that only go outside, or can even go both on potty pads AND outside.
How much grooming is required? Yorkies are pretty high maintenance in the grooming department, especially if you want to keep the coat long. They have to be groomed ever 4-6 weeks and definitely need to be brushed with a conditioning spray at least once a day.
Welcome to YT and don't hesitate to ask any more questions you have! That's what we're here for!