I always make the Bookkeeper, Broker, and Manager the same dwarf, and it's always worked reasonably well.
I do the same. The broker is usually too busy to trade though. Not that it really matters as most caravans get slaughtered by goblins anyway.
I always use the 'burrow over the trade depot' trick and add the broker to it whenever a caravan shows up. Then you just want one with a strong work ethic personality trait so they aren't always on break/drinking during trade time (like my last fort, with Urist Diskon, the Broken Broker...)
For Bookkeeper, it seems like it doesn't really matter who you give it to, since they spend like 1 day a season doing it once it's set up, so their stats don't even seem to affect it much. My current bookkeeper / manager has his organiser and record keeping skills at "V. rusty" and "rusty," so that must be an indication of how little time they need to spend doing those tasks, if it's not even enough to prevent rust. (Bookkeeping set to highest accuracy, tons of orders through manager)