Colin - From my experience collecting the set the toughest singles to get in PSA 4 or better are #34 Ridpath and #32 McNamara. From the HOFers the toughest card to acquire is going to be the Lalonde #36. Both Ross cards #8 and #12 will set you back a fair penny as will the #15 card of Cyclone Taylor. Not all PSA 4's from this set are created equally so get a good scan when you're buying. Good luck!
From my experience putting this set together, the toughest card was the Lester Patrick. I finally found one at the expo in Toronto after a year of looking. Ungraded, off condition cards on ebay also tend to be higher priced than most other hofers. Good luck on the set!
I recently started working on this set myself. It all started with the Cyclone Taylor RC. I had wanted one for years, and finally bought one. After that, I wanted to keep going.
There are a number of C56 cards on eBay right now. A bunch of them are in eBay stores, so they will be there for a few weeks or longer. I'm also working on a PSA 4 set, but 2 of my cards are PSA 5. I completely agree with not all PSA 4's are created equal. Normally I wouldn't ever touch anything lower than a PSA 7, but with this set you have to make certain allowances. Some PSA 4's have rounded corners and possibly a slight crease on one corner, but others have paper loss or a nasty crease that takes away from the eye appeal of the card. So far, it has gone well for me, but commons at this grade usually go for $150 and up! On the other hand, the full set only has 36 cards.
Good luck with the set Colin. I'm working on one as well - PSA3 grade and I agree with Albie - the Ridpath and McNamara are tough commons. The Ross cards, Patricks, Taylor and LaLonde will cost you but are all classics.
These cards show up on eBay frequently but are not as common as c55s and some other pre-war issues.