OT - who can tell me about dogs, especially training dogs?

 Look like a little Lucy twin!

Kelly,
Everyone gave you good advice on dogs so I'm not going to add any.

When we got our lab was about 2 years old, we got a cat (first one for me, it was my son's birthday present). We kept her in the back room at first so she could get use to us and Wags would sniff her under the door. When we let her out, the first two months were war. When he saw her, he would chase her and she ran. After about two months, one day she decided not to run and they've been best friends ever since. We had an old dog too at the time, but she couldn't move very well so she just barked at the cat and Claws learned to make a wide path around her.

Good luck. Dogs are very comforting and I couldn't imagine not having one in my life.
Jan

I should add that after telling him no a few times with the litter box, he hasn't touched it in years. And he stays in the house all day while we're at work.

A true service dog will be trained to do, or not do whatever you want it to.
I would also check with your insurance company to see if they will pay for some or all of the cost. You may get your psychiatrist to write a letter of recommendation.
Look into training facilities for puppies that aren't quite making the grades. You can get them at a discount price. They could be good "students" but for some reason or another something slight is keeping them from graduating.

I think volunteering at your local shelter is a great idea to get you acclimated to dogs. They can be much more work compared to cats.

The love you receive from a dog is different than cats. Dogs are more aggressive with their love where cats are more subtle.

The kind of dog you need doesn't require it to be a large dog, in fact, I think a smaller dog would be best because they are easier to take places. Also, a smaller dog would probably be more accepted by your cats because it is at their eye level and might feel less intimidated by it.
Just quickly off the top of my head, I think spaniels and toy poodles are small, smart dogs.

Thanks for the suggestions.

I think for some of the tasks I need it to do, the dog will need to be a certain size.  For instance, it needs to be able to reach the light switches so it can turn on lights in dark rooms for me.  Although possibly I could locate some light switches at a lower level than usual if necessary.

Someone suggested I look into guide dogs that did not complete their training for some reason.  Apparently sometimes a dog gets through a good bit of the training and then is found not suitable for working with blind people but can still work as a service dog for someone with slightly different needs.

I find that having a big dog helps me more than a little dog would, but your mileage may vary. Rio works as "crowd control" for me. He gets between me and any group of people. He is so friendly and wiggly they never notice he is blocking them.
As someone who went through some violent trauma, I appreciate walking with a big dog. He is friendly, but he has also made it very clear he is watching over me (once when hotel housekeeping came barging in, once when a guy went in the woods behind my tent to "excuse himself".... Rio made both of them go back where they belonged, quick. ).

Rio actually has trouble with light switches because his paws are so big. I haven't tried training him to flip it with his nose, I just have him walk in the dark rooms with me, with him just in front. I would love to get the big "smash pads" as I call them, for my lights, so he could just hit the big pad.... but for me, him by my side helps.

Just with Petsmart training he knows how to open / close doors (push doors, of course, not round knobs) and push a buggy. He knows "touch it" (paw - what I used to help teach him to open / close doors), "point" (nose- when a gentler nudge is needed - ie to not scratch the car door closing it for me) and "back up" (as far as I motion for him to go).
I've taught him "move", "turn around" (switch positions of his head and hindend), to carry a back pack and to pull a load (which he doesn't do since his knee surgeries). He knows "go look" means for him to go to the window.
He also is a good "Farm dog" - I can tell him "go that way" "go this way" and he helps me round up rebel roosters. He will "watch the gate" to keep chickens from running out.

As much as you love to read and research, you will most likely do a great job training a dog. I think volunteering at the shelter is a great idea! And you may even meet your new best friend there

I was also thinking a larger dog would be good for crowd control.  Dogs can be taught to position themselves in such a way to provide a buffer between you and other people, but a little dog may have more trouble doing that well.

I was thinking of changing my light swtiches to those ones that are more like touch pads than the switches you flip, if that is easier for a dog to turn them on and off.  I think having the dog walk into the dark room with me would work often but I also need him to be able to turn on lights if I am sleeping in a dark room and have a nightmare and he needs to be able to do it for me then.

I have heard great things about farm dogs and all the work they can do.  It's really amazing.

 

If you have the time and patience, you can actually train your dog for a lot of service dog tasks.  I would definitely do the series beginner, intermediate, advanced at a pet store or trainer.  Our Petsmart lets you repeat for free, so I did. 
  Once you and the dog have the basics, and know how to work together to learn, your only limit to training is your imagination.   Rio does crowd control on "stand here". I am trying to work him into just hand signals, but in the mean time folks just think I am putting him there for them to pet him. He technically probably isn't doing the actual command a professionally trained dog would do.... but if it works it works.
  Even though I am not physically impaired, I taught Rio to "Brace"  - stand strong, don't move - so when I fall I can use him as a prop to get up. This was SOOO helpful when I was SMO and couldn't get off the ground without an act of God or a crane.