The wheels might look a little smaller than normal if the bike was meant for 27", but if it was a 700c bike then the wheel/tire combo of a 650bx38 should be pretty close to a 700x23. I think it would look just fine.

As for brakes, like you said, you'll just have to measure to see. If it was built for 27's and could accommodate big tires then they might not work, but if it was 700c and built with tighter clearances then they probably will (given, I don't know reach numbers on those brakes).

I'm not sure about the fenders, but going back to a 650b tire/wheel combo being close to the same size of a 700x23, then I'd think that fenders for 700c wheels would be what you want.