great bike and i ride a 620. the stem shifters don't complement the bike, so maybe DT shifters or my favorite: bar end shifters.
Or real thumbies on trekking bars. 😎 It looks like the original cantilevers there, which fit those 27" wheels perfectly. There's a thread here about converting to 700c, that explains the positives of 27" wheels & tires. You (meaning the OP, assuming he gets the bike, lol) might decide not to switch, after reading it. 😉