Personally, I think you can generally get great buys on some very classy, top quality, steel bikes, which will update with a real minimum of tinkering, if you target late 1980s era. ~ STI didn't come out until 1991, and 130mm spacing for 8-spd (same as 9&10-spd) rear hubs was new for 1989, but otherwise, much of what we're accustomed to now, had already worked it's way into bike technology.

But... if you want to go with something older:

I've "Hot-Rodded" even a 1960s Peugeot PX-10 to 130mm rear spacing for current 9/10spd hubs and tapped out the previously un-threaded (French style) rear derailleur hanger to accept any standard modern 10 x 1mm derailleur bolt. So, it now runs on an all new drivetrain... EXCEPT for the crankset. It still has old Stronglight cranks - just because I'm funny about springing for fitting that French threaded shell with a Phil Woods BB simply so I can use a more modern crankset - which isn't really needed, at all.

But, I think the real gems are British bikes. They already are tapped to the same threadings we still use for the BB & headset and (unlike French bikes) use a 22.2 quill stem - for which you can always get an adapter post to which you can even clamp a threadless stem (if you absolutely MUST). Top-end 1970s Raleighs (International & Professional for example), usually even had 27.2mm seatposts.

Older Schwinn Paramounts are also easily updated.

And, Japanese bikes are a nice choice, too. Just one thing to watch out for on them would be use of JIS (Japanese Industry Standard) sizing on fork crowns & head tubes and possibly uncommon seatpost diameters.

Older ('70s, etc.) bikes commonly had brazed-on rear derailleur cable stops or guides routing cables ABOVE the chainstay. But, you can get around this with a clamp-on chain stay stop to guide the cable below... and, you can also easily drill and tap a single bolt hole beneath your BB shell to attach a modern Campy/Shimano derailleur cable guide under there. A clamp-on Campy down tube shifter band, with the levers removed, will be fine for attaching cable guides/adjusters for STI/ERGO cables. I know, this all sounds like a bit of a bother, but I think that if you don't re-orient your cable routing, you may have a bit of a mess criss-crossing over the STI and especially ERGO derailleur cables after they exit your handle bars... [BUT, I may be completely wrong about this].

Also, check out the distance from the brake mounting holes on the frameset to the rims. Chances are, you'll want to use modern dual pivot brakes, too. Some much older bikes may have a very long caliper reach which might be a pain to accomodate with even something like Shimano's longer reach BR-R600 calipers. And (again, on pre-80s bikes), I expect you'll have to drill out the frameset for recessed brake mounting... but it's generally really simple to drill out a hole from 6mm to 8mm, anyway.

Maybe this is a stupid generality to make, but I think that really the higher the quality of a vintage bike, the more likely it was to have gotten the newer "stylings" (like recessed, shorter reach brake calipers) which gradually trickled down to most other models, later.