I have a 20mo Lab and have just taken on a 11wo GSD.
We got a second dog because our Lab is prone to anxiety, but is a lot more confident when there is another dog around. And he loves other dogs.
Plus points - Lab is very happy to have a buddy and does seem more confident. Plus I love the fact that the two breeds are so different, so may balance each other out.
Also, I feel less guilty leaving the pup for the odd hour as he has company.And the pup gets socialized by playing with the lab.
Minus points - I feel I am neglecting the Lab a bit at the moment as the puppy needs socializing, so getting carried on the school run, etc.
Plus when I take the lab for walks the Pup feels left out. Training is a lot harder as if I am trying to train the pup, I have the Lab showing off that he knows all the commands and expects a treat. 
I'm also a bit worried as our Lab, like yours, has the patience of a saint so doesn't 'tell the pup off' when he is a bit boisterous. Once Pup has had his 2nd Jab I will be taking him to meet more assertive dogs to learn some manners!
Time wise, once the pup can go for proper walks, two is no more work than one. Cost wise, obviously all our costs have doubled 
I have a 7yo DS and a pre-school DD so I am tied to the house anyway, which is why we made the crazy decision to go to a two dog family.
I would definitely go for a breed that compliments yours.
If you have any specific questions, feel free to ask / PM me. 