I always look at the paint. The bike may be dirty but if the paint underneath is in good condition it will make any restoration easier. also if the paint is in good shape the bike will probably not have been ridden as much. You can get a beautiful result by deep cleaning the frame , rubbing out with white polishing compound , then either waxing or, like I often do, clear coating with Rustoleum Automotive clear. (warning, never spray this over fresh paint. Disaster lurks as it will alligator the finish if the paint is not THOUROUGHLY dry).
Also check to be sure the seat post and stem aren't stuck. that can be a show stopper. Beware of heliomatic rear freewheels. best to just replace the wheel rather than mess with them. Heavily rusted bike - stay away. wheels with severe hop in rim - look for new wheel. cotteed crank - you better have access to a cotter press.
Old French bikes in particular can have compatibility issues. They can usually be bought cheap though so buying a parts bike might be a good idea.
When things work out though you might just end up with something like this 1962 Pug I restored for the Bike Exchange a few years ago.
