>>1773752Group set refers to the components fitted onto the frame, primarily the braking system, drive train and wheels.
You also need to look out for quality tubing, Reynolds, Tange, Prestige, Columbus etc, any bike that has a frame made with any of those tubes, is fitted with any of the above group sets and is clearly in good low mileage undamaged condition is a bike worth buying, they are light for steel bikes, extremely versatile, easily adapted to alternative roles - bike packing, touring, commuting, trail and gravel riding etc etc etc. Plus parts are abundant, compared to modern equivalents fairly cheap and much more durable.
Picrel cost me £150
Like anon said - go to retrobike forums and ask there, they have amazing knowledge and resources.
https://www.retrobike.co.uk/forums/ 