Forget about making your own battery unless you want to learn about how they work.
That motor is already quite heavy. It will need about 20 to 25 amps to work properly. From recycled 18650s, you'll be lucky to get 1C or 2 amps per cell, which means that you'll need to build a pack 10s10p, 12s10p or 13s10p. A modern Ebike battery can do it with 4p, so less than half the weight.
To make your battery, you need a lot of equipment. A decent balance charger will cost £60, then you need ballance boards, charging leads and jigs. You must check every cell because one bad one can make it catch fire. That's very time consuming. You need to think a lot about the routing and protection of each wire. One touch and you'll be seeing smoke and flames.
How do I know all this? It's because I've done it. It was a great learning experience and taught me a lot about batteries, which was my main goal.
The kit will do the job. It's main drawback is the heavy direct drive motor. If you want power or torque, you'd be much better off with a geared hub motor or a crank-drive kit. Don't forget to add about £30 to £40 duty on it. That takes it to about £300, which is a about what it's worth. You'll need a DNP freewheel for it, as with any rear freewheel hub-motor. That'll cost another £25 to £35. You can't use the ones they show in the advert, which have too low a top gear. The full-width throttle is not really suitable for a bicycle, though you can probably cut it in half to make a half-width one. You can only use the brake levers if your donor bike has cable brakes with their own levers.
