You don't have to charge each pack separately. You can charge them all at once, and you only have to get them over 4 or 5 volts, then the regular charger will work. Since the sellers were riding it around recently, it shouldn't take much. The hard part is disconnecting the BMS board and connecting a charger directly to the cells. As Powell has noted, you don't want any "bouncing", or messy connects/disconnects. I think the the safest way is to pull the whole battery assembly completely out of the hub housing, so you can get at the fuse on the back. It is much easier to get at the screws on the fuse than the screws that connect the cells to the BMS board.
I'm glad the bike went to someone on thist list-congratulations!