Sounds like a nice upgrade (one of many!) to me. I've heard that the Trek 720/620 cantilever studs were positioned a bit differently from modern standards and only worked with a small list of cantilever brakes. If the studs have been repositioned to the modern 700c standard I'd call that an improvement. I can't say whether the fork or frame has been compromised by the re-brazing but it's not a hard job and any semi-competent welder could do it without trouble. I wouldn't be overly concerned unless the seller seems like a total crook and based on the bike, I'd trust him. Finally, for $450 you best decide quickly, that's a good deal for that bike.