Get a brushless hub motor if you can. It has less friction and allow you to pedal like a normal bike if you feel like some exercise.

As for staying in shape/lose weight, it doesn't work for me. It's too tempting to push that throttle and away you go. I didn't lose any weight all that time. I'm now going back to my normal bike, for commuting, and use the e-bike when expecting a really steep hill or a very hot day.

My heart rate has increased more regularly now that I use my normal bike for medium hills. Try your normal bike first , if the hills are not too steep, because your fitness will improve.

PS. Lithium battery is very handy as you don't need to worry whether there is enough charge to get you home. Mine last more than a week before needing a charge for a 10 miles return trip commuting daily - medium hills .