What are the advantages of a building an electric bike from a kit vs. prebuilt
First, check your local laws. As others have mentionend, in some countries (EU), maximum power for an electric bike is 250W. Go beyond that and the vehicle will be considered a moped and will need license, insurance and and license plates. Your local laws might also require an assist-only option for your bike to be considered an e-bike. That means usually a pedal sensor, and those can be difficult to install especially with exotic or outdated bottom brackets.

Selecting a kit is not necessarily easy. If you're going to front mounted motor, check that brake type is compatible (disc or v-brake) with the kit and that fork spacing is suitable. E-bike kit should come with brake levers with a cut-off switch so the motor stops pushing immediately when braking (problem with some e-bikes).

Kits can be had cheap, but installing them takes time, effort and experience with bike mechanics. If you got all three, go for it. If you need to get up and running quickly, an entry-level e-bike is probably better option.