Zjrog, Like you I'm retired and can't push a bicycle like I used too. Be careful with the older aluminum road bikes. I've known several 200 lb guys that broke the head tubes on 1990's era Specialized and Trek bicycles. Like you I am wary of carbon fiber. I believe the epoxy matrix is not the right material for bonding the carbon fibers together. The epoxy is not tough enough to shrug off real world handling dents and dings that a steel fork does. There is a Carbon fork for Clydes discussion under Clydesdales that has the following quote from a Zinn article that is referenced in the discussion: From Deda:
Carbon lasts longer than metal.Only love is stronger than carbon.Bonding is a different story.I believe that a good glue (epoxy) can last for 2000 hours of work, or about 800 days, not in continuous daylight, and below 35 Celsius.Whenever a carbon “part” has crashed, even if you cannot see a failure, if there is any reasonable doubt about having surpassed the elongation limit, the part must be replaced.
–Fulvio Acquati
Deda is a carbon fork manufacturer and this is their comment about carbon fiber fork useful life. Gives you pause to think. I believe that a different tougher matrix for the carbon fiber will be developed that is much more real world worthy and durable. Until then approach with caution. Buying used carbon fiber bicycles has quite a bit of risk associated with the bicycles crash history. Experts in the field say that you should have the frame and fork ultrasound inspected before buying. It will be interesting to see if these bicycles have long or short careers as they age out and fall into the hands of second and third owners and even the homeless. There are still massive amounts of early steel mountain bicycles being used by everyone including the homeless. I want to wish you luck with your weight lose and your search for the perfect bicycle.