Hi and welcome.
Before you build decide what you want to do with it. That will make a huge difference in what you need to build.
In my case I ride mine to run errands but i live almost downtown. My longest trip for bike parts or groceries would be six miles round trip. I can do that riding it like a motorcycle so I don't worry much about pedal assist. Most of my pedaling is done on startup and hills, otherwise I just ride mine.
Now as to a kit it is much cheaper to put your own kit together. That said you run into about three general type propulsion systems. One is the hub motor. They are a bit expensive, and I had bad experiences with mine. I don't recommend them but some guys do so give them a thought. The second system is an electric motor and that runs the rear wheel like a scooter. You can buy a kit from Currie I am told. The third is a friction drive system. Okay you can also give the pusher trailer a thought.
The cheapest thing is to build your own. An electric drill is about the most exotic tool needed. Well you need a way to cut the metal to size. A hack saw will do that for you. The truly easiest is the Rhino drive(,my term) With that you use a scooter rear wheel, mounted over the rear bike wheel. The power comes from a scooter motor, which hangs off the side, secured to the scooter wheel axle, it uses a scooter controller and throttle. All those things are easy to find on ebay.
There is a dealer on ebay named stewie. he has the rear wheel assembly and the motor mount. Most any currie three bolt motors will fit it. They come in all kinds of watts. the best is 500 or more. Those motors will run at 24v 36v or even 48v. I find 36v the best. but you should match your controller to the watts and volts you want to use.
If you want I'll give you detailed directions to make the build. I think this forums sister an energy forum still has my directions for a pusher trailer on it. The pusher is a good system but I little more complicated to build.
Whatever you decide good luck. The biggest mistake I have made over the years is starting with too small a motor. pedal assist sounds good to tree huggers, but it is a pain trust me.