For me the reason to buy a vintage bike is because it is precisely that. The original braze-ons, paint, and even components are part of that appeal. So the question you should ask yourself is, do you actually want this Mercian, or do you want a "vintage-looking" fixed gear with new parts and paint?
If you've found a bike that tells a story, don't obscure its heritage. This frame can easily be cleaned up, waxed, polished, and built with periodish parts as a fixed racer. A lot more appeal there than flashing it up with new everything.
Just my opinion, of course.