I've been trying to change my answer to this question for the past week or so, but it appears that I can't. For me, it turns out, the answer is a combination of buy and finish the build with whatever I have on hand.
My two builds in progress are an '84 Pinarello Gran Turismo and an '82 Trek 614. I bought the Pinarello as a complete bike with SunTour Superbe components, but I wanted to rebuild it with new Campy parts. I bought the Trek as a mostly bare frame (headset and bottom bracket) and intended to move as many of the Superbe components to it as I could. I just needed a downtube shifter clamp and brakes. I stopped by the ISO thread here and otg helped me out with the shifter clamp, and a pair of nutted Tektro R539 brakes turned up on craigslist today, which I'll be picking up shortly. So I guess the Trek build qualifies as "buy" but mostly it just all fell into place easier than I had any business hoping it would.
The Pinarello is a different story. I need to sell another bike to finance the build I want, so it's going to be a while. I had the best intentions of being patient with this. I stripped it down this weekend and put the frame aside. But now, I find myself having the "need the last 12 pieces" blues, and my patience isn't up to the test. I was playing with my planning sheet today and I realized that my parts bin holds everything I need to put this thing on the road...with a Tiagra drivetrain and some other assorted parts that will leave it looking very much like a frankenbike. It won't be pretty, and it's nothing like the build I had planned. On the other hand, I will be able to ride this bike while I'm waiting for the cash to come in to rebuild it the way I want.
Really, I can't even convince myself that this isn't reasonable.