1. ebikekit, a well respected vendor, sells only hub kits. Ask either Jason or Dan, both honest, knowledgeable individuals, on their chat "forum" about their kits.
2. Make sure that you ride a bike equipped with a 350w, 10 ah kit on a hill before you buy one. I'm 175 pounds with a light bike and it doesn't offer much assistance on steep >5% hills.
3. The mid drive kits from em3ev are priced competitively compared to domestic vendors even with the extra shipping cost (I have no affiliation with any vendor).