Articulation and good diction do not necessarily run counter to "regional mannerisms or colloquialisms". Not a bad rule of thumb is sing the way you speak, but sing clearly - there's nothing more offputting than somebody trying to sound like somebody else.
As far as building a repertoire, sing what takes your fancy - if a song is 'old and hackneyed' it's probably because it's a good song, so why ignore it?
Jim Carroll