Depending if you can find one and their price...I would recommend a Jamis Eclipse or equivalent (never owned one...but almost - I was pipped at the post). They are a steel frame, but the lightest steel frame you can buy - prob equivalent to some alloy bikes.
Be careful buying 2nd hand alloy, they have a limited life compared with steel frames - their useful/recommended life is actually only about 5yrs before brittle and stress fractures settle in. Carbon has an even shorter recommended life.
Personally I think the components are the most impt, its the running gear that keeps you moving. I would look for something a little older (don't be too proud) but with decent running gear. Probably try for a bike with Shimano Tiagra components or even Shimano 106. Try to avoid a bike with "Sora" components as they are entry level. It would suffice, but components are what will give you the headaches and maintenance issues, not the frame.
An older bike with decent components is more likely to have had a decent rider maintaining and appreciating a solid bike reliable bike.
Having said all that, if you are going for alloy you probably want a low milage bike - and definately not crashed.
Experience is a hard teacher...she gives the test first and the lesson afterwards.
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