TRGarin- I too would steer away from carbon fiber (aka plastic) bikes, especially used ones. You don't know if they've been dropped or not which can be a VERY bad thing for a carbon frame.
I am almost 78 and have owned carbon (new), aluminum, and steel framed bikes. Two months ago, I decided to settle on a new steel frame because it is easier on my bones. The tires are 700 X 42c and so the ride is very comfortable. I ride it for pleasure but mostly for commuting to work. It is a touring bike actually. It is a SALSA MARRAKESH that was about $1800 new. The ride feel might be compared to riding in a Cadilac instead of a Corvette. So, if you're not too interested in speed (been there, done that) you might want to take a look at this bike on Salsa's sight. It was kind of a getting back to basics thing as in (steel is real) when it comes to frames. Also, it does come with drop bars, but I have it set up so that the saddle is at the same height as the top of the drop bar. I am also 6' and the 57" frame fits me perfectly. Best of luck to you.