My advice woud be find a mate or acquaintance who knows bikes and take them with you for advice. And you guess right about the mileage - bike mileages are usually much lower than cars. They age worse if not looked after, as many of the components are made of lighter, more fragile components (or are simply badly made) and materials.
Mileage isn't the be all and end all. Look for service history, signs of being crashed (scuffs, replacement panels etc). A lot of people do their own maintenance, to varying standards. If modified, are the original bits available?
I could go on, but a mate would be the best bet.