As one who works in rescue (with Dachshunds), I wanted to agree with several previous posts.
If you love dogs, you’ll do whatever it takes.
Adult dogs will sleep most of the day, and if they have something to entertain them the rest of the time, they’ll be fine.
Getting two is not a bad idea. In the house, they don’t take up that much more space and they will be much happier together. However, your costs will go up: twice as much dog food, vet bills for vacinations, bills at the kennel, etc. Anywhere you can take one, you can take two, (if they are well behaved, but you were planning on being pro-active with behavior, right?) and when you can’t take them, they won’t be lonely.
I agree that the dog should be crated, or at least not given the run of the house. People think of crates as cruel, but they aren’t. Dog’s prefer to sleep in a den, and a properly sized crate (one he can stand up, turn around and move a few steps in) is actually very comforting for a dog. If you don’t have room for a crate, at least don’t let the dog wander the whole house. This has to do with pack dynamics. Where he is all day he will think of as his area. The house is yours. If he’s visiting, he’ll be well behaved.
Adopting an adult dog is a wonderful idea. They are housebroken and calmer. Sadly, many shelters put dogs over 5 to sleep automatically, because it is so hard to place them. Rescue groups know their dogs, and will keep them rather than place them badly. Sadly, many dogs are given up for reasons having more to do with owner behavior than their own. Moving is a big reason, for example, so you have a really good chance of getting a dog without major behavioral problems. In any case, the rescue people will let you know.
Dachshunds, despite being often smaller than cats, can look upon cats as animated games of tag. What we often recommend is a two-tier system. Let the cat have some high places (say, the tops of several bookshelves with some strategically placed two-by-fours, a comfy bed, a scratch pole, etc.) where he can hang out without having to worry about the dog. I realize that Boxers are taller dogs, but the principle is the same. If the dog isn’t cat-agressive, they will work out a way to live together.
Also, a friend who is a dog-trainer tells me that male Boxers are the undiscovered treasures of the canine world: sweet, intelligent, loving, eager to please, and well-behaved by nature. You could do a lot worse!
E-mail me privately if you want information on training or have any questions.