Not sure how things are in Chattanooga, but you will still have to exert yourself to climb hills, just way less than before. Also, REI lets you test drive these for free, so I would definitely try it out before buying!

350W motor, so you'll hit that 20mph limiter on the flats, but maybe won't sustain that up hill. It's better than a 250W motor, but not by a huge margin.

Not too great on steeper hills due to having a hub motor, but the trade-off is hub motors are really cheap.

Good plan! The first time I rode my old bike after some years of neglect and a few extra pounds, I was quite alarmed when I got off the bike and found that some parts of me had fallen asleep. I have a new respect for making sure the seat is a good fit.