Since you're a lightweight at 120 lbs, do focus on butted framesets. Your 531c would be a great tubeset, 753 even better. As would Ishiwata 022, any triple-butted offering, Tange Prestige, and Columbus SL. Truly light and flexy if you're a big rider, would be Vitus. I believe their butted steel set is the lightest.
Skip the $700 Moser, it's overpriced!