Quarantine is actually better if it lasts for 30-60 days in separate parts of the house and then they are introduced slowly. Many illnesses do not show up for longer and a week will not be long enough to make sure both birds are healthy.
Next yes budgies do usually like having other birds around but if their people provide them with enough interaction they do not have to have a bird buddie. Sometimes it can cause a budgie to be wilder instead of nicer if they have a friend. When you are the "flock" for your bird there is a greater chance it will bond with you and stay tame. When adding another bird there is more of a chance it will bond with its bird mate and will not want as much to do with the humans. If you are wanting to have a tame handled budgie I definitely suggest not getting another one at least until this one has fully bonded with you and enjoys your company. Then you could slowly introduce a well socialized budgie that is already used to being handled by people.
If you are wanting your budgies more for looks and in cage interaction or are not able to spend much time with it then getting it a buddie could be a great idea. If you do get another budgie after then quarantine period introduce them in a neutral space slowly so they become used to each other do not just go out and buy another one and stick it in the others cage or they can become aggressive and actually injure each other.