Yup, batteries is complex beasts my friend
Buy a cheap one and the cells could be a no name copy of a better cell. Their current rating could be low so giving a decreased range.
You need to look beyond headline statement figures.
What cells are they (manufacturer/type)?
What configuration are cells in?
What's the maximum draw of your bike's controller?
Will it fit the frame and where?
Look at post #3 from D8veh.
You need to :
1 - Get the bike
2 - Decide on the kit and show it to the guys, they'll let you know if it'll do what you want.
3 - Find/be guided to the right battery.
WALLET ALERT!
You may need to adjust your budget along the way, so as not to end up with something that disappoints so much, it ends up in the back of the shed.