We became a 2 dog family over the summer, so I thought I may try to answer some of your questions. One thing that is nice is that our "old" dog really helps to show the puppy how to behave. I let them both out to use the bathroom together and the old dog is trained to come right back in so the puppy follows right along beside him. If let out alone, the puppy tends to want to play and wander so it is much easier with the two. The older dog (an 11 yo lab mix) also seems to have perked up a bit since the puppy came along and she seems to enjoy having his company.
Unfortunately, our puppy has not been easy to potty train and we are seeing lots of accidents. You get used to how easy your big dog is, so there is definitely an adjustment to having a puppy who has accidents and chews up everything in sight. It can be really aggravating at times!!
I have to feed two seperate foods to them due to their ages, which is a little more time consuming. I feed them in seperate area, because our big dog eats everything in sight and would eat the puppy's food. (Our food and vet bills have doubled).
Both dogs are very well socialized, so there was essentially no adjustment needed when we first introduced them. They both tend to be submissive, but the puppy has been very content to let the older/bigger dog be a little more dominent when they play. However, the older dog is very laid back and I have seen the puppy grab her bone and take over her bed with no problems.
I think the most important thing to take into consideration is the temperament of the existing dog and the one that you are getting. Both dogs should have experience with being around other dogs with no issues. If it is possible, I would even try to arrange a "play" date between the two dogs to make sure that they will get along well.