Hi Warren, welcome to the forum. I can't begin to relate what I've learned here.
However, you will get advice from some that don't relate to being happy with slower pleasure riding. In 8th gear my trike will only go about 21 MPH, but in 1st gear it will climb a 15 to 20% grade at 6 to 7 MPH with light pedaling and the motor running efficiently (i.e. not lugging and heating up).
I think any hub motor is a good choice for a mid drive, since many are designed to run at a low RPM, somewhere between 300 and 400 RPM at 48V I think. The downside of the motor I used is it's weight and outside diameter (had to shoehorn it into the space available), but otherwise it is perfect for the job.
One issue that I didn't get right until completing the conversion was the gearing between the crank and the motor. Initially I had 48T on the crank and I couldn't keep up with the motor at WOT, and since I like to pedal all the time I was running the motor below it's efficient speeds. My motor and crank are on the same chain and I ended up with 16T on the motor and 56T on the crank, which works out good for this motor and me.
Good luck with your build. I will enjoy watching it develop.