When we added Abbey to our household (she was #2), there wasn't much difference financially. She was from a rescue, and had been spayed and had her shots and everything already though. We did of course have to buy her a collar and leash and food dish, but all of the toys were shared.
We thought a submissive female would be best as well, and it was at first, but she started to challenge Kito's alpha status after a few months. They had a few scuffles, and one pretty bad one which resulted in her needing surgery and stitches

As far as companionship goes, we wanted her for the same reasons. Kito had lived with other dogs previously as well, but I think he did get used to being an only child for a while there. But he did always have a constant companion--they stayed together when we weren't home, and played together and slept together.

Kito isn't the best example though, because he's a spoiled brat. I know you're looking for the difference between 1 and 2, but when we brought #3 home (Riley)--then Abbey had the playful companion that she needed, and Kito could go off and do his own things when he wanted to as well. Riley and Abbey have never fought, and still stay together when we aren't home, and sleep next to each other every night.
If you find a relationship like that one, you are lucky, because you have succeeded in doing what you wanted to do--find a companion for your dog.

I wish you good luck in your search and making your decision--in my eyes 2 is better than 1, but multiple dog households aren't for everyone.

Keep us updated please.