Which is planned first--the songs or the musicians? Each group of instruments can have its own songlist, so you want to decide first what kind of music you want--Traditional, Beatles, Journey, Sinatra, Irish, Bluegrass,Jazz, or your own special theme. Guitar, piano or saxophone might be the more authentic choice rather that string quartet or harp.
Talk this over with the musicians before hiring them so you both know what's expected and possible. It is reasonable for musicians to charge extra to transcribe music off YouTube if it is not in print or to buy the music and to rehearse it. Not all songs can be done on all instruments. We want to play your favorite songs, but do want to play the music which sounds the best on our instruments.