Be careful with “cheap”. Quality costs. A year later you might find expensive repairs.
Bafang motors are costly but good quality. Good batteries are expensive, there’s no way around it and most fires seem to be from the cheap ones.
But a kit is the way to go if you have mechanical skills. And it’s easy to go overboard on estimating “needs”. What’s your goals? It’s ok to not know or wanna go fast or just commute to school.