An adaptation of the musical is a great idea, but you're gonna cast non-singers you might as well just do a movie based on the legend of Sweeney Todd.
Non-singers are only bad if they don't work. There are plenty of cases where unique sounds that are far from acceptable in normal standards can produce solid work. This can be especially true in the world of musicals, where often the stress is on how well the respective performer embodies the character, not on how well they sing. The best version of "Send in the Clowns" to date is Judi Dench's (as recorded on HEY, MR. PRODUCER!), and she can't sing a lick, but the emotion drips from every word.
In Burton's SWEENEY TODD, the cast's unique voices here each brought something new to their roles, and in almost every case, it was worthwhile. Given the performers' dedication to their characters, I felt like the music actually gained something from having even deeper performances applied to it, even when the technical side of the music suffered a bit. Their casting certainly fits with Sondheim's mantra of "actors who sing, not singers who act." Remember, Sondheim's the guy who cast Glynis Johns in A LITTLE NIGHT MUSIC. Everyone involved in Burton's SWEENEY is
miles better than her as far as musical talent is concerned.
To look at HBC, she may not be able to sustain a phrase like a great singer (though that really only takes its toll on "The Worst Pies in London"), but man, does that voice haunt me. Its whispery, unearthly tone does have something quite beautiful about it. As a result, I've never heard Mrs. Lovett's longing for Todd in "My Friends" or her desperate pleading in the "Final Sequence" done quite as effectively as her renditions, even though hers isn't the most successful from a musicianship standpoint. There's a reason she was Sondheim's favorite during the recording sessions.
As it stands, I'm more than happy to set my SWEENEY TODD soundtrack alongside the OBC recording and the New York Philharmonic recordings.