Richard,
I have not used any older 'c' lenses, black or chrome with the P20. My oldest lens is a 20 year old CF 60mm which is fantastic, the others being the latest CFE IF 40, 100CFI, 180CFE, all of which are as good as it gets. However i have over the years used many 'c' lenses with film, some of which are not as good as the later ones, something digital would show up more.
Given the pricing of 'CF' lenses now which are cheap, i would be inclined to at least stay with them and not the old 'c' style which are in general not as good as their CF/CFI/CFE counterparts, allied with the fact that the shutters are very old now, and not as reliable as the ones used on later lenses.
Regarding the 503CW, given that it is crucial to get correct registration between body and back, far more than film, i would be tempted to buy a new or late lightly used one, which will give you more chance of perfect body alignment, and thus digital back registration. Also the 503CW body has a gliding mirror system which stops the cutoff in the viewfinder of lenses longer than 150 and also when using extension tubes. Of course this will be less of a problem with the P20 viewfinder crop as opposed to say a P25.
As a guide to some really good lenses available at reasonable prices, may i suggest any of the following: 50CF FLE, 60CF, 80CF, 100CF, 180CF.
Regards, Gary.