As far as what size string to get, that is a personal preference. If I was starting out, and had a vintage martin guitar, I would take it to the best guitar place you can find, and talk to them about it, because they can set up your guitar for a certain gauge, so there are no string buzzes, and so that you can have them check the neck for you and just do a general setup. They can help you decide what gauge string to use. The same goes for what kind. You will have to try them and decide for yourself what sounds, and feels best. But go to a good guitar place and have them help you, and setup your guitar. Its not very expensive to do.