Basic stuff: rear hub motors are a pain when, not if, you get a flat. so either go with a mid-drive or make your rear tire impervious (tannus liners or something equally protective). avoid proprietary parts and oddly sized wheels (looking at you, radwagon). make sure you have a local shop that will support you, not just for mechanical issues but also electric. many shops won't work
on electric bikes if they did not sell them.