"The LBS says 'depends on the spacing on the bottom bracket, but shimano sora and tiagra fit pritty much all frames'"
Sounds like typical modern LBS speak for 'I know sod-all about old bikes.'
Of course the BB will accept any modern BB set, nothing's changed there dimensionally since Adam was a lad! As said above, it's the width across the rear dropouts that is important. To get modern (ie 10 speed etc. - or even 8, 9 or 10) stuff to work they need to be 130mm apart. Yours will be 120mm and therefore suitable for 5-speed or 'Compact 6'. Again, as stated above, try to get suitable period fittings for it. Plenty to choose from on here, at jumbles, e-bay even, over time and potentially a lot less expensive than a new groupset and wheels and ultimately hopefully more satisfying in the long run.
Today is the yesterday of tomorrow.