I own 80's vintage bicycles, and I own vintage cars; on both cars and bikes I try to stay as much as possible with the original equipment found on that car or bike that came, or would have came, stock in the year in which it was made. Personally I would scour e-bay and find Campy parts of that vintage and put it on the frame because most Mercian's came with Campy. The older group sets from the 80's were actually more reliable then newer stuff so you don't have to worry about dependability. I have several bikes from the 80's and not one time do I worry I have obsolete parts, the parts are still available, but they rarely break anyways.

Optionally, though not original on a Mercian, you could buy the complete Suntour Cyclone components that actually work better and last longer then the Campy and still be period correct, but be way less expensive then Campy.

Either the Campy or the Suntour stuff can be found either unused or virtually unused on E-Bay.