Actually, walking big dogs isn't really that hard.... I saw your concern for that and I was already planning on adding to what you'll want to get. Definitely, if you are getting a bigger dog, get an anti-pulling harness. (not a choke chain, or one of those collars that digs into their necks to make them not want to pull)... I still can't figure out how the anti-pull harnesses work... the just go on similar to a regular harness, and under the front legs. The dogs don't pull. Even on rollerblades, I can stop 3 dogs (over 200 pounds worth) without too much effort. If you're having any trouble with the dog destroying things, chewing on the corners of furniture, tearing up garbage, etc., get a shock collar. They're a little on the expensive side, but work extremely well. My vet recommended one to us and assured us that it's humane; it's unpleasant, but far more startling than painful (and what pain there is, doesn't linger). Make sure, though, that the dog doesn't recognize what the controller is for the collar and doesn't link you pressing the button to the shock... otherwise it'll understand that it can't do the bad things when you're around, but if you're gone, it can do as it pleases.
And... finally, before you completely discount the idea of 2 boxers, get input from boxer owners as to how much extra care they need... Some dogs need to be brushed daily, and their extra care can really take up more time than you may be willing to spend on just "care" things. If the care of boxers is relatively simple (provide food, water, outside to #1 or #2, and walks), then taking care of 2 is no more difficult than taking care of 1. And, you'll be rewarded with twice as much affection from your pets.
btw, 3 dogs is really hard... I only have 2 hands to pet them with when they greet me at the door every day.