As to what to top off with...super Tuscan is a "catch-all" name for any blended wine that is not Chianti.
Found on the web...
"Some super Tuscans do contain Sangiovese, either 100 percent or in blends. But others are made solely from Merlot (such as the famous Tenuta dell’Ornellaia Toscana Masseto), from a blend of Cabernet Sauvignon and Syrah (Riccardo Baracchi Toscana Ardito), or from even more unusual blends, like an amalgamation of Petit Verdot, Cabernet Sauvignon, Syrah and Merlot (Argiano Toscana Solengo)."
Practically any wine will work, this is the only reason I make Welches grape wine, to top off with. At least I know that the finished product is all mine.
As to the sulfites in commercial wine...they won't affect your fermentation at all. The percentage is way too small.