I once owned a motobecane (gave it away for free in 1990!!!)
very simple bike to start on to learn how to dismantle and rebuild with new cheap parts.
if it has been stood for a long time then rust can get into the bearings and first time you ride the bike in anger you could get a massive (expensive) failure
or like has been said before rings could be seized. crank seals could have perished too.
I understand if your not into the mechanics side and just want to fuel up and ride you will want a quick fix.
but sometimes stuff isn't half as hard as what you imagine and there is loads of help/ info online/here
and there is so much satisfaction to be had doing it yourself.
although when it comes to my car/van I am happy to pay someone else...