Hello. I think an older dog would be a wonderful choice. But I might first suggest that perhaps you might consider volunteering at a local rescue center. That way, you would have a chance to assess your friendship level for a particular dog. If you are in the second half of your 70s, I would suggest a dog that won't be too peppy. A dog that is 8 or 9 might be a good age for you. It will need to be walked, but it won't walk too much faster than you might want to walk. I am speaking from experience. We rescue cocker spaniels. Normally we get dogs that are 9 or so, because the older dogs are not as sought after as the younger ones. Last summer, we got one who is maybe about 7, and he tires me out! He needs an extra walk every day. I'm in my early 60s!
So I have two suggestions, actually. Try visiting your local rescue and see if they would like help walking their dogs or having someone play with them to keep them socialized. And that way, you might have a chance to get to know the dogs better. If you only have foster rescues in your area, then fostering a dog that you think you might be able to bond with might be a good way to test out the circumstances.
You might want to do some research on the need of certain breeds. A basset hound, for instance, might have difficulty doing stairs as he ages. Cockers do okay, but as ours age, I do tend to have to pick them up and carry them up and down. I also, as each ages, eventually have to get down on the floor and feed them by hand, because each of our dogs has gone blind, and it's very hard for them to find their food without getting it all over their faces!
I think a dog is a great idea. I know what you mean about loneliness. An older dog would love to bond with you. They usually have had good families that just can't care for them (or afford them) any longer. We have two at a time, and sometimes three. We love them all. I don't know what I'd have done without having the dogs to care for when my beloved mother died on Feb. 28. They have saved me by making me keep active and attentive to their needs.