I've got a garage full of frames and I'm building them up with various levels of components, depending upon the frame and its intended use. Not period correct but nothing unusual for the life of a good frame.
There are lots of good climbs around here with very fast and sometimes curvy descents. My life is in the hands of my bike when I'm going 40+mph so I make sure everything is in proper working order.
On the bikes I've found and fixed up for friends, I've always repacked the bottom bracket and replaced cables and housings. New brakes for one bike, new pads for another, new coaster brake. I've not managed to break even on any of these but that was never the point.
As for bikes to flip, if it requires mostly time from you and you've got the time; go ahead and give it. If you put some time into it, you can get most old brakes to work just fine; same with derailleurs. Bottom brackets and headsets benefit greatly by getting new grease and ball bearings, both of which are pretty cheap.