I agree, and two of the most important issues that could come up are:
1. 27" wheels versus 700c wheels on older bikes (see
this link)
2. Freewheel or freehub, which will require different drop out spacing if you want to change from one to the other (but steel frames can be reset). On Dawes I think the transition from free wheels to freehubs occurred around early 90s. If you wanted a true to original restoration with an older bike, it's still possible to get good quality free wheels, just not so much choice anymore
Neither of the above are particularly difficult to solve.
1. Very deep drop brakes will take up the difference if you want to use 700c wheels in a frame built for 27".
here or
here if you want dual pivot braking efficiency.
2. "cold setting" a frame to chage the drop out width is quite easy with a length of M10 or M12 threaded rod, nuts & "penny" washers.
Depending on what you want to do, what can be more challenging atre the presence or lack of the braze-ons on the frame: Very old ones lack a derailleur hanger, necessitating a separate bolt-on hanger for modern mechs - which reduces the effective length of the horizontal slot in the drop-out, which then makes removal and refitting a rear wheel a pain.
Also the very old ones used band-on down-tube shifters, whereas the newer ones had brazed-on studs. If you want to convert to STIs or bar-end shifters the brazed-on stud style is a lot easier to convert. The only adapters for band-on style that are now available are just monumentally fugly. There used to be some nice looking chrome ones, but they are rarer than hen's teeth now - and they don't have provision for cable adjustment.
And missing on some older frames are the bosses for bottle-cage mounting.
More modern Galaxys (late 1980s onwards) have the studs for cantilever brakes, which not everyone likes, although the original BR-AT50 Shimano cantis they fitted in the 1980s are pretty good even by today's standards.
I think I've done just about everything one can possibly do with a 531 framed Galaxy.
Mine is a late 1980's fully converted to modern 9 speed STI
My wife's (same vintage) has now been converted almost completely back to original configuration and is in daily use for commuting
No 1 son has a 1970s vintage which has modern wheels/cassette/rear mech (8 speed) - but operated by DT friction shifters
Daughter has 1970s vintage which has modern wheels/cassette/rear mech (7 speed) - but with off bar-end shifters
No2 son has a 1990s one that was bar-end shifters but is now 9 speed STI.
He also has 1980s on that we've just turned into a fixed - he rode to school on it this morning
- and we have Super G tandem !