8FUN (Bafang) has the majority of the mid-drive DIY conversion activity; their BBS02 & BBSHD are well received and seem to have decent track records. I've used a BBS02 for the last year+ and it's performed perfectly. Look at Luna Cycle for the best prices. electricfatbike.com has more about them than toy'll ever want to read. Biggest consideration (IMO) with an AM (or most full suspension bikes) is where to put the battery.
Get a bafang bbshd they are very durable and dont have the controller issues the smaller bbs02 had with being in the wrong gear and pulling to many amps for the controller to handle. Lunacycles now has new small cube 18650 batterys that you can fit anywhere if you dont have room in your frame.