I think building up a repertoire is something that happens gradually. I find that when I practice a lot (which I do), it sort of crystallizes out what I like to play. Sometimes, I'm surprised at what songs I keep going back to. Sometimes I'll leave things for awhile and then go back to them later or even much later.

It's better to play and/or sing three songs well than 100 that all sound the same. Things sounding the same is probably something everybody has to struggle against (unless they don't care). A certain amount of "sameness" is unavoidable, since it's the same person, but one can try to get some variety into one's music.

I don't think there's anything wrong with "old", but "hackneyed" is a problem. If it sounds hackneyed to you, you probably shouldn't be singing it in front of an audience. However, no matter how hackneyed it might seem to one audience, it will be new to somebody. One could perform for children, for example.

On the subject of accompaniments, I agree with most of what Don Firth says. I have a somewhat different perspective, since I've had to stop singing because of physical problems with my voice. It's forced me to concentrate on playing. It's too bad about the singing, which I enjoyed, but my playing has gotten a lot better.

One thing I learned from singing lessons is that one sings better when one isn't playing an instrument at the same time. I know a lot of people do and not everybody likes to hear this, but I think it's the truth.

I also think folksongs in particular often don't gain from being accompanied by guitars, accordeons, autoharps, pianos, banjos, bouzoukis, bodhrans, mandolins, ukeleles, trombones, etc. Much as I love instruments of all shapes and sizes, I think harmonizing them and accompanying them the way they mostly are nowadays turns them into something different from what they were before being collected, arranged, put into books, "revived", etc. I know this is a contentious subject and I don't have the ultimate answer to it, either.