Susan, first of all I am so sorry for your loss. I know and understand how difficult it is losing your precious baby. I lost Bella almost two months ago and the pain is still present.
After my other kitty died six years ago, I wasn't ready to adopt again. I had her for almost 16 years and I did not want her to feel replaced. Four months after she died, I started fostering kittens since there was a huge need for foster families. Two years later, Bella came to me as a long term foster. She was born at the shelter, put into two different foster homes. Her second foster mom adopted her mom and three siblings, another family adopted two kittens. No one wanted Bella because she had a heart defect and it would be too difficult when she died. So I happily took her in. She immediately crawled on my lap when I took her out of the carrier. "She had me at meow." She helped me foster many kittens thereafter. Sadly she lost her battle on February 2.
I want to address your concern about fostering. It is a huge commitment and a lot of work, but fostering gives kitties or puppies, cats or dogs, a start in life that they would not have. The only life they would know is the life of a kennel. Fostered furry babies are usually adopted first because they know how to be held, how to play and how to love. However, the final decision would be yours. I cry every time I return them for adoption but I know if I adopt, it might be difficult to continue fostering. Bella was unique in that she loved all the kittens. Most cats would have a difficult time to adjust with new little ones in the house.
I would not call adopting a foster a "failed foster". It can be difficult returning them and some families do decide to keep them. It's not a failure at all. I was lucky that I was able to continue fostering after I got Bella and that she was so good with other kittens. I ended up adopting Bella because she was labeled "unadoptable" and there was no way that I could give her back after having her long term. She was my heart and my life, and I would do it all over again if I could.
Only you can decide what is best for you. You can give it a try and see how it goes. If it doesn't work out for you, you can always return them to the shelter and they can find another foster. There have been a few times that I got kittens that were in another foster home before coming to me. The shelter does provide food and flea treatment (and litter for kittens). If medical care is needed, they cover the cost of that as well. So there does not have to be any cost involved. I always provide food so that there is more for the shelter and for other fosters who can't afford to buy any.
I wish you all the best with your decision. Follow your heart and do what is best for you. Maybe a senior foster would be something to consider.