Very nice... yes, I remember the mini backlash about painting Arabesque type "lugs" on top of the real lugs, but this is beautiful. I can't remember seeing one in this color scheme... really like white bikes. And I gotta say... I'm liking the lines of that S-Works stem just fine, and I'm thinking I would want it to be black just as is. If you really want a "wet white" stem, then there's the Deda Zero 100 stem, but they're kinda hard to find now, and maybe too heavy on the logos. Is that a -10 for the S-Works? Ever think about just stripping the logos off if they bother you? At this point everything looks like it's coming together perfectly.
Will you be swapping out the BB for some Campy Ultratorque cups? If you do, might want to think about a light facing of the BB shell first as it doesn't look like that's happened there. Not to remove material, just any paint that is covering the shell face.
And is there a story with the headset... might the cups have been replaced at some point? They look very angular, like the Acros on the C60, but I don't remember the C59's having that. The Colnago headset cover that sits on top looks like it doesn't quite match up with the cups, and they're usually pretty good about that. Just wondering if there's a story there or not. Actually, now that I'm jogging my memory, Colnago did use a wedge type headset cover when they first switched to the Acros headsets. Wouldn't be at all surprised if the original headset cups were the Acros model with that "wedge" compression system, which forced apart the two piece cone underneath the stem to press against the headset and bottom of the stem. It was not a very good design imo and would often come loose. They went back to a more standard arrangement (but still Acros) after a very short time. In fact, now that I think about this more, I'll bet this is the story... The original headset was in fact that very Acros headset, which also came with the wedge type top cover. But I'll bet the previous owner swapped out the top cap for the one that is on there now. It's not a perfect fit, but it allows the more conventional bung in the steertube and pre-load being applied from the top cap above the stem. Yes, that's what I bet happened.
In any case... this is going to build up really nice. As for tire sizes... I can fit a 25mm tubular (using the new 2015+ Boras). Clinchers can be a different story depending on the rim being used. Those 24mm tires on those rims are looking just fine in the rear... but I found the clearance issue on the C59 was more noticeable under the fork. Now that Specialized have some nice tires in the 24mm width you could always use those. But a 25mm clincher on an even slightly wider than standard width rim might pose some clearance issues, especially if it is a Conti 4000sii, as they tend to sit the tallest of any clinchers I've used. Width is not an issue on that frame... lots of room.
Enjoy!... Looking forward to seeing it built up.