I figure Ebikes like you are talking about draw around 30-40 watt hours per mile at 25 miles per hour, depending on tires, hills and rider weight.
I would go with something like a
36V 20AH V2.5 LiFePO4 Battery Pack because they come with a charger and a BMS that shuts power off before you damage and destroy the battery pack.
If you are going to use a trailer then I would add more batteries to the trailer and plug them in when the trailer is connected, for that you could use cheaper lead acid batteries and an isolating diode to keep the packs separated.