I have the bolt and wouldn’t hesitate to recommend it. To answer your questions/points of interest directly:
1. If distance is the most important thing for you I’d recommend getting the speed sensor that goes on your rear hub as the package deal, which should only be a modest increase in price. While the head unit alone will do the trick, from my understanding of how GPS units work the head unit will measure your path, but occasionally check with the speed sensor to address any gps drift that may occur over time and therefore maintain the highest accuracy, especially in areas of poor gps coverage such as tunnels or forested areas. I might be very wrong on this though, and would happily be corrected.
Correct on the speed sensor information, though I'd say only get it if the GPS track as recorded is showing cut-offs at tight turns or errors due to dense tree coverage. I use my Garmin without speed sensors on 2 road bikes with no real loss of mileage. I do use the sensor when. mt. biking as it better tracks the really tight and twisty single track I ride. I also use it on my go fast carbon road bike as it reduces lag for indicated speed as compared to a GPS. That's useful for maintaining a steady speed in a group ride.