I am so sorry about Basha, and sorry for your sad loss.
Though it does sound like you did the right thing for Basha. You really worked hard with this seizure issue she had, and took it to the point I would have done, to the point at which there is no shadow of doubt that you made the right decision. When medication ceases to work, or nothing can help the situation, that's when it's wise to stop, and let them go.
Spanky will also be grieving. Replacing Basha very quickly with another addition to the family may backfire quite badly.
Spanky KNOWS that other (pup? dog?) isn't Basha....and would not necessarily welcome an "intruder" on her patch, especially if she feels vulnerable, which she is bound to feel after her own loss.
My dog passed away in June. She had a faithful old friend, she had known this dog and they were very close for more than 10 years. This was a neighbour's dog, not mine, so there was a limit to what I could do to comfort him. After her passing, he lay down and I really thought he might die as he was so obviously grieving. Every day I went to him -controlled my own grieving emotions when I met him, so as to appear as normal as possible. I comforted him, but did not do it in a very sad way. I tried to interest him in playing games (as we always used to do all together) It took him three weeks to a month to come around, but I did those things every day.
One thing which made him worse, strangely enough was when he caught my dog's scent from a door-mat in my porch, where she always used to lie. Another thing was when I mistakenly played a video of her barking on my phone. He ran away, and I realized this had upset him.
If he didn't come into contact with either the sound or scent of her, he was better. He could "move on".
I have a theory that they instinctively sense they have a better survival chance if they "move on" emotionally from grief..
That dog is perfectly fine now. After the first month, he returned to his normal self. (though he is a very old dog, and may pass away soon himself.)
If you are having any problems coming to terms with your grief and loss, there is a bereavement website where there is a lot of understanding and kindness:
http://forums.rainbowsbridge.com/?forum=150830
I am so sorry for you and your family but it sounds like you made the right decision for Basha. It doesn't sound like she had much quality to life and often this is the last act of love we can do for our beloved pets.
I am sure Spanky will miss Basha not being around. You can leave something with Basha's smell on it around for a few days and give Spanky some extra attention. She will know you are upset. The dynamics in the home will change but she should adjust. I would not advise getting another dog just to get another dog. You or Spanky may not be ready and you would want to take the time to decide if you want another dog. Do you want a puppy or consider adopting one of the many dogs that need loving homes? What breed do you want and what size dog? If you find one, you may want to try to have Spanky interact with it to see if they would be compatible. So, lots to think about and I think you all need time to grieve. When the time is right, you will know.
Hope this helps some and sorry for your loss.