I've done a couple of total running gear upgrades to old frames and I think a couple of points are worth bringing up.
When you do a total redo of the mechanicals you get the chance to really go to school on how your bike is put together and how to fix and adjust shifters, brakes etc.
To me, this is very valuable, no matter how many things I do on bikes I can always learn more and it brings you closer to being a fully functional rider who can solve problems on the road as they arise.
Also, if you load up an old frame with top quality and well chosen running gear and you find a frame you like better, you still have all that gear to do your next build. I have a very nice set of wheels that have been on three builds and have enough life to see a few more..money well spent I'd say.
This is my latest, an 86 Nishiki (I had this in C&V) built it in two days from a totally stripped frame, did one test/adjust afternoon and have been riding for a week now with zero issues.
Mike