I would buy a kit, and put it on an existing bike. Then if something failed, you could replace that part. If it seemed to not be good quality, you could replace it with an equivalent part from a different manufacturer.

It also depends on what is available in your country. In some places you can buy decent ebikes, at reasonable prices, which probably have spare parts available.

I would probably go for a rear wheel geared hub motor, so you can replace the wheel if it fails.