I would strongly recommend a Specialized Turbo from 2015/2016. It's a rear hub rather than mid-drive, but after 3 years on a BBS02 mid I prefer the rear hub (quieter, zero maintenance). These bikes show up on ebay with light use fairly regularly.
If you're thinking about doing a build on an existing bike, check out ebikes.ca. Superb engineering, pricier than the chinese stuff but worth it.
Good luck!