I don't have an e-bike at present, but I too will probably get a kit and add it to an existing bike. I will probably go for a geared rear wheel hub drive. The first one will be road legal. If I get two, the other one may be for off road. Adding a kit to a bike normally works out cheaper. It also makes it easy to replace parts if you need to. Parts on manufactured e-bikes are normally more expensive. I will probably get a spare battery.
Just stick with well regarded and known brands and avoid proprietary stuff (which ends up costing a lot more down the road). You'll have fun building it and be able to deal with any issues from a standpoint of knowledge as well.