Karla,
I'd leave well-enough alone! We got Finn (the sheltie) when Brady (the mutt) was showing signs of slowing down, hoping that Gus (the schnauzer) wouldn't miss Brady when he went if there was a little friend here for him. Gus tolerated Finn when he was a puppy but after Brady died, Gus took a distinct and rather aggressive approach to Finn.
I think if your dog is happy like that I'd wait and get the new dog after she's gone.