If you have several years to dedicate to this, go right ahead and have fun. Otherwise, I strongly and respectfully suggest you consider how to at least start from where Rosegarden already is. You could also help the Lilypond team on their work to create a GUI/WYSIWYG system for their tool. Even then, given your goals, plan on years in the saddle.
--p (5+ years on audio sequencing, and counting)
