If you only want a dog because of allergies, you should re-think this! Some people do well with dogs and don't have allergies, and others are totally miserable with the dog hair and dander! What type of allergies are you talking about? I think you need more information on this topic.
The breed of dog is important in terms of the age of children in the house, the family's lifestyle, and the expenses involved. You can look at a pure-breed dog (generally expensive) or a cross-breed (mutt). Some dogs are just not good with kids (such as Dalmations), and some dogs have been overbred and have health or personality problems. That's why a mutt can be a good fit. We've had both a purebreed and a mutt, and both have worked out great. Our son was 7 when we got the first dog. I'm not sure that 4 year olds understand that a dog is not a toy, it's a living thing that needs care and also doesn't always want to play or be bothered. Senior dogs can be very loving and grateful for a new home, but they don't always want to play actively. They can be lovely companions but if you don't want to deal with death, then think long and hard about this. Any pet will be one that does not outlive you, however.
And every child will tell you that he/she will walk and feed the dog. Do not believe this. No child will do this for more than the first week. It will be YOUR job, every day, morning noon and night, in rain and snow and sleet and burning sunshine!
Do not adopt any dog if you are not 100% convinced that you will NOT return it! That is absolutely unfair to a dog, especially one who has been given up before. It's not a dress that you return to a store. It's a living thing with feelings and confusion about where it's home is and where it fits in the family structure. You should go to a breeder or a shelter where the staff absolutely knows about the dog's history and temperament. And do not get a dog if you are hoping it will die before you return to your home country!
A small dog is not good with an active child, nor is a dog with fragile bones (so don't adopt a greyhound). However, a large dog will not manage well in a one-bedroom apartment. All dogs must be taken out at least 4 times a day (often more), and in all kinds of weather. If you have a 4 year old, that means you have to take your child with you because you cannot leave a 4 year old alone. So be prepared that when the dog has to go out, you have to get shoes and a coat on your kid too, and get everyone out the door before the dog pees or poops in the house! They also must be protected from extreme heat and extreme cold. They absolutely must be trained. Big dogs need to run a lot more, some more than others.
Your friend who adopted the dog who snapped may not have been well-trained herself in how to deal with an adopted dog moving into a new setting and not yet used to the new family. Dogs need to understand their place in the family, and to know that they are not in charge. But a dog adopted from a shelter must be dealt with in a special way during the adjustment period.
It's so hard to judge expenses - you must spay/neuter, you must vaccinate for rabies and other diseases (distemper, for example), you must license your dog with your town/city. They absolutely need annual visits, but just like people, some dogs get sick and need care in between annual physicals.
Food costs depend on the size of the dog - big dogs eat more. Little dogs tend to live longer, and so do animals who have been spayed or neutered. You need a leash, a collar, 2 bowls (water & food), appropriate toys, training treats, a brush, and probably a crate or at least a bed.
You can bathe a dog in the bathtub or take it to a groomer (could be in a pet store or a privately run grooming facility). Dogs don't need baths all the time and too much can really dry out their skin (so then you have to go to the vet to have them treated).
A good shelter will know the temperament as well as the medical/living background of the dog. A good shelter will have you fill out a lengthy application (ours was 5 pages) and ask you questions like: Where will your dog sleep? How will you train your dog? Has anyone in your family ever had a dog? Who is your vet? Who can we call for references? How much do you think it costs to support a dog for a year? What will you do with the dog if you move? What will you do with the dog if you go on vacation? What would cause you to return the dog to the shelter?
If you don't have the answers to these questions (and it doesn't sound like you do) I wouldn't adopt a dog. One thing you can do is volunteer at a shelter, help walk and feed the dogs, learn how to relate to them, and learn about training and personalities and breeds. You'll know soon enough whether you want to continue with the idea of adopting a dog. If you don't have time now to do that, then you don't have time to adopt one either.