I'm glad you're really thinking about if a cat is right for your family. Cats are awesome & fairly easy but not for everyone.
1. It sounds like it is already pretty difficult for him to visit... 2. Litter boxes don't generally smell unless your cat has a medical issue or you don't clean frequently. I have the TidyCat Breeze system and don't smell anything if I scoop daily, and its heavy pellet litter doesn't get tracked around. 2a. I am strongly against outdoor & indoor-outdoor cats, especially if you have coyotes around - it is so dangerous for them. Can you get the kind of dog door that only opens for the dog (they wear something on their collar I think)? If you absolutely can't keep the cat inside 100%, make sure to get collar, microchip, proper vaccines etc. 3. Learn to clip the cats nails and do it weekly (vet can show you), provide a variety of scratching posts. No declawing. 4. No declawing is even more important with a dog-chasing cat. You need to introduce slowly anyway and keep them separated while you're not there for a while, but the claws will help the cat put the dog in its place. Also provide a cat tower or high places it can escape to.
Ok....you would absolutely need to address the dog door issue because a cat can get hurt/lost/eaten if you have coyotes. I would stick to an indoor only kitty.
Cats do scratch stuff. You have to trim the nails and deter them from the moment you adopt them. They're very playful, adorable and pains in the butt when they're really little.
Litter--well, it has to be scooped very regularly. But as long as you take care of that, it does not smell.
You do have to make sure the cat and dog will get along, but every animal is different so it's hard to say until you actually have the cat. The cat may love the dog or hate it. My mom has a husky that was best friends with our cat, groomed each other, slept together etc. When that cat passed away, she adopted a siamese kitten. The kitten was absolutely TERRIFIED of the dog and hid in the towel rack in her bathroom except when it was meal time. It was not a good situation, so I took the kitten because she was the same age as my new kitten. My two kitties are BFFs.
That being said, if you're committed to training your kitty, then do it! I love kittens!
I brought a new cat into the house last year. I had a cat that died earlier so I knew how the dog was going to respond to the cat. What I didn't count on was how the cat responded to the dog. For the first hour, he was rather wary around the dog but now you'd think that they were siblings. Last night, the cat had his paws around the dogs neck, washing his face!
The rest of your issues can be dealt with, other than your friend. Litter boxes don't stink if they are kept clean and scooped regularly. Cat claws can be snipped, and you'll need to provide something for them to stretch and claw on. If you have coyotes, then you don't want to consider having an outdoor cat so you can get a dog door that has a signal on it that will only open with the dog.
You can't imagine how a cat changes your life. They are funny, cute and if you just adopt one rather than walk in a shop and buy one, you made the best decision ever for both you and the cat.
Don't be afraid, the dog will play with your new cat and will just have lots of fun together.