We went from one to two and don't regret it but all of your questions are valid and it's good that you're thinking it through beforehand.
We had hoped that the second dog would be nice company for the first when we were out. Our second dog came from a friend who is a breeder. She was three years old and being "retired" and re-homed. I was trying to make arrangements to get her here (she was halfway across the country) when my son flew there and got her to surprise us for Christmas.
So fun! But.....it didn't give us a chance to acclimate them to one another. I wish I had had the chance to introduce them on neutral territory. There was a lot of excitement. She was frightened in her new surroundings AND she was in heat. So when he approached her she tried to take his face off. LOL !!
Well not that bad, but she gave him "what for" and that stayed with him. She's really a very mellow dog but it wasn't a good first meeting.
So plan that well.
It did change our dog sitting arrangements. When we went from one to two it was too difficult for our back up sitter (usually my son is available) but I've since found another one who is WONDERFUL and I have a friend who lives nearby who is always willing to run over and give them dinner and a piddle if I'm out and running late.