Fit and reliability .... Fit doesn't need to be millimeter-perfect, but if it hurts to ride, you won't ride.

Reliability is important ... obviously you cannot ride a broken bike ... but that goes for Every bike.

Since you are buying used, you hopefully will only be looking at bikes in very good shape, and will know what to look for. Otherwise ... yer scrooged.

I Assume you will only buy bikes in good shape, so reliability wouldn't be an issue. If you are buying a bike and are not Absolutely certain it is in ridable condition and will last a while ... well, it's your money.

A reliable bike which hurts to ride will last forever ... in your garage.

A reliable bike that fits well will last just as long, but will need maintenance ... how much depends on how much and how hard you ride ... but you will ride.