The most important thing you need to do when considering an electric bike conversion is to check the laws and regulations where you are. It's much easier to go afoul of what's allowed when buying a kit. A lot of the time if you buy a bike the manufacturer has been through the process of ensuring what you are buying is okay in your location. There are different options available in different parts of the world. Buying a kit puts a lot more of the responsibility on your shoulders.
The laws governing electric bikes are hyper-local. They change depending on what country you are in but, for example, in the US different states have different laws and it can even get more local than that. Check the size of the motor, does it have a throttle, what is the max assisted speed, as they all change the requirements.
Depending on the system you end up with there are different requirements. Some systems will make trails off-limits. Other systems might be okay but require insurance. Make sure you have an understanding of what you are getting and what that means for riding it in your location.