I am 6'4". I think this would be the right size. My road bike is 61cm so I think this is correct.
If you are 6'4" you should be fine, I would still do a fit to be sure to remove the "I think" out of the equation. Large bikes are a tough sell used, so I would start off low with out pissing the guy off. Based on the setup in the pix the current own strikes as a bit of a dope so look over the bike thoroughly. You will need a new stem though, probably 110-130 with normal rise like a road stem. do the fit & good luck CX starts soon so get on it. :-0