I am majorly considering getting a dog in my current apartment. I love them! I think you shouldn't be discouraged... training is key! Whether or not you are working, you want your dog to be well-behaved. You will have to take at least 2 weeks of just you and puppy time, as this is essential to building your bond and establishing yourself as packmistress. I recommend reading "How to be Your Dog's Best Friend" by the Monks of New Skete. They raise German Shephards in New England and have written the best dog owner's advice book I've ever picked up.
As other have mentioned, make sure you socialize the dog and that he is acclimated to a stream of unknown men coming in and out of his domain. Barking, be it excited or aggressive, isn't good. It'll annoy your clientele and bother your neighbors, causing loss of business and unwanted attention.
My dog won't be a big one, either. My complex has weight and breed restrictions.
Personally, my work day is scheduled between 11am-6pm and I see, max, 4 clients in a day and hardly ever back to back. I don't know how you operate, but I'll have time to tend to my dog. If I recall correctly, the number of hours they can hold their wee is almost equal to their age in months. If you got the dog from a reputable breeder, they should be no younger than 8 weeks-- 2 months. That's two hours. I'll be at my place or near it; letting him out wouldn't be an issue.
Crating will likely have to happen, but I expect my dog to be good enough that I won't have to do it all the time or with every client.
Don't get a cat. They just aren't the same. No sub for puppy love! Besides, I'm deathly allergic to them (as are many other people), it may be crazy (I've been randomly attacked by nutjob-hellspawn felines! And before I started FBSM my last cat *loved* guys, and would crawl and rub all over any male company... sweet, but not if you don't like cats), and litterbox smell is disgusting.
Fish are WAY more maintenance than you'd think.
Dogs are like KidsLite, but remember they are pack animals (NOT BABIES) that have been bred to want to make their owner/master happy. As mentioned, research breeds that will fit your lifestyle and personality, learn what you can about dog behavior, clean up after him (like OCD-style), and get ready for obe of the mpst fulfilling relationships you'll ever have!