I am looking for a substitute for my commuting bike. It is a cheap second hand general store bike. It do the job but just don't worth keep maintaining. But at same time I have to keep my bike outside for 10 hours a day in place where several bikes where already stolen (attached with cheap bike locks but still a fact). So I am afraid of leaving a decent bike there.

I checked second hand bikes pages and looks like there is a lot classic second hand old road bikes for sale really cheap and from big brands.

But I am not sure how much work a bike from 90s need and what I should check if I am buying one or even if it is a good idea.