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I would avoid a used carbon frame. Without knowing the complete history of the bike, you're probably taking a gamble. Maybe it was never dropped, but you cannot be sure. At 50, especially since you've been off the bike a while, I would avoid carbon and aluminum altogether. Both are generally going to be very stiff. While they can be highly efficient, ride comfort may suffer.
If you're comfortable with doing a bit of wrenching, and know your size, check out
BikesDirect. You can get lugged steel frames in your budget, or even a titanium frame with SRAM Rival if you can go a bit over budget. Both will offer performance with excellent ride quality.
Oh, the Motobecane sold by BD is not the same Motobecane you used to ride, but they're still great bikes.