I just completed my first go at this, @
Ed. It was easier than I thought, but you're right, there's not a lot of top-down overview information out there. Here are the basic considerations:
1. Wheelset/speeds. The only way to keep the current wheelset is to use a 7 speed setup. As others have mentioned, you can go with Shimano RSX, older Sora, Tourney, or Microshift. The latter setup is really affordable. You can get the brifters, FD, and RD for about $90 on Amazon. Shimano 7 speed freewheels are also insanely inexpensive for how well they work. Like $12 on Amazon.
If you change the wheelset, that opens up a whole world of possibilities of 8, 9, 10, or 11 speed groups. Velomine.com has great prices on wheels, or the forum classifieds, obviously. Shimano 9 speed stuff seems very easy to find used at good prices. Ribble in the UK has the best prices on new groupsets. Be aware that Campagnolo 8 speed is not compatible with 9, 10, or 11 speed wheelsets and vice versa. Likewise, Shimano 11 speed is not compatible with 8, 9, and 10 speed wheelsets and vice versa.
2. Spacing. If you stay with 7 speed, the 126 spacing will be right on. If you go to 8+ speeds, the spacing will be 130mm. This is not a big deal. You can cold set it if you like, but really you just need to spread the stays a tiny bit when getting the rear wheel in and out. It's only 2mm per side.
3. Cable stops. Does the frame have downtube braze ons? If yes, you'll need some basic downtube barrel adjusters to route the shifter cable. If not, you'll need a clamp on stop.
4. Brakes. Is the frame drilled for nutted or recessed calipers? If you are switching the brakes for dual pivots to match the groupset, you may have problems with recessed brakes. There are workarounds, or you can drill the frame. There are also nutted options available ala Tektro or Dia Compe.
5. Fit/reach. Modern brifters are really long, so modern handlebars have compensated by having shorter reach with drops that sweep back farther. You might try something like the Soma Highway One bar -- comes in silver with a 26.0mm clamp area, so quill stems fit.
Overall, I was surprised how idiot-proof the whole process was. You just have to route the cables and use the RD's barrel adjuster to make sure the indexing matches up, and off you go.