Sweeney is a great choice! It is a very, very goood show. I'd say go for orchestra seats, isnce I've been told(never sat there, but I am in 11 days) thata it is a show that is much better close up.
Your othere choices also sound fantastic. However, if you are a Sondheim fan, you might want to check out the new revival of Company coming in the fall. It's directed by John Doyle in the samme style as Sweeney, and people who saw the show in Cincinnati said that the concept works better in Company than in Sweeney.
I am a firm believer in serendipity- all the random pieces coming together in one wonderful moment, when suddenly you see what their purpose was all along.