I, like you, have knee problems. I have torn Meniscus in both knees. If I ride distances, I need to wear knee braces or it actually makes my knees hurt worse.
I, also like you, rode a pedal unicycle when I was 13 years old but have not been on one since. I took my first EUC to Sam's Club and held onto a shopping cart rack in the parking lot after they closed and rode around it one time. By the time I got back to the beginning, I had it and took off. I rode about 600 ft that night. The next day I took to a local bike path of about 5 miles in length. I rode the whole distance without stopping one time because I did not know how to stop!!! OOPS! I took the whole ride back practicing starting and stopping because I quickly realized it was super important around people and eventually cars.
After 3 days of riding my generic wheel, I went and bought a NineBot E+ and LOVED it! It's a very capable wheel. Your price is very good! I've now moved up to a GotWay MSuper and love it even more.
Others have answered the other questions well, so I'll avoid those.
But I will say that I also own a NineBot Mini Pro, and I think it is MUCH better suited for what you want to do. The amount of control on the Mini Pro is a lot easier to use. You can stop and start without fear of falling off, and around walking traffic, it's a breeze. The Mini Pro is NOTHING like the Hover Boards, and I've owned a few of those too, so I know. The Mini Pro traverses obstacles very well. It can go up and down wheel chair ramps easily, across grass and gravel, and even go over bumps quite well, something the hoverboards can NOT do well. I've even taken my Mini Pro off road and it does well there too since it has a computer that helps keep it straight and level when riding over bumps and angles. The NineBot E+ is also able to do all these things, but it has a much larger learning curve and even when you are very good, you still fall off occasionally. Running around foot traffic on the NineBot E+ is very possible, but not until you are comfortable riding 1 mph or less, and that can take some time.
The other thing is that a Campus is likely to be more willing to allow you to cruise around on the Mini Pro over the E+. Especially while you are learning. While learning, many people have quite a few falls and mishaps, and it is perceived as being more dangerous because it simply looks that way. The Mini Pro is super easy to learn, 5 mins tops, and it's SUPER stable.
Good luck in whatever you get. Either way, I hope you take up EUC also. It is probably better suited for off campus riding though, which you will LOVE!!! Off Campus, it runs circles around the Mini Pro. On the Mini Pro, I've gone 15 miles on a charge at around 10-12 mph, so it's still capable, but the E+ can do it so much easier when you know how to ride it.