Is it worth buying a second hand classic road bike for commuting? 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.
A (decent) bike from the '90s would not be significantly different† from (a decent) one only a few years old except for a small weight difference and possibly lacking brifters, which are de facto standard on road bikes these days. This statement is of course excepting top-of-the-line superbikes made of carbon fiber and dragon's blood.


Check it for stuff like rust in hard-to-see places (small bolts etc.): This is a good sign of how well (or rather poorly) the bike was taken care of. If there is no or minimal rust, it's probably been taken care of well enough.
Look over the frame for any signs of crashes: Big dents, cracks, fissures, etc. 
Check if the headset wobbles or not: It shouldn't, and if it does, it might need to be replaced, which would be very expensive.
Check how worn the chain is: If it looks horrible and probably has never been changed, that's a warning sign.
Check the teeth on the front chainring(s): If they are starting to look like shark fins, the chainring will need to be replaced. 
And of course, just ride the thing and see how it feels.




† "Significantly different" means from a mechanical/engineering perspective: IMHO, some of the ugliest bikes I've ever seen were from the '90s.