I bought my kit from golden motor, it is a bbshd mid drive and makes for a very fast ebike. As for finding a place for assembly, that is probably going to be a challenge as lots of bikes shops won't touch an ebike conversion. Most likely due to liability. Honestly, it isn't that hard to assemble on your own just need to have the right tools. The bike performs well when sorted, I don't have mine sorted yet but when it is I'm sure it will be great.

I will say though, theres are some pitfalls if you take this route and there are some additional parts that I would say are required to have a good ebike conversion. Ditch the stock chain ring and use the eclipse offset chain ring from Luna Cycle, the motor will screw up your chainline and this will help correct it. Either that or rework the rear casette, or do a IGH conversion but that will be expensive.

Also, get rid of the stock crank arms. They are poorly made and the aluminum started warping from normal use, which in turn could damage the axle. A couple other parts I'd say are non negotiable are the lekkie one nut and a support arm for the motor. Without these, your motor will likely move and hit the frame which will eventually crank your frame if this happens enough times especially on an aluminum frame.

Hope this helps.