1. Ready to fly (RTF) are good if you just want to launch a rocket with no effort. If you want to learn about model rocketry then a skill level one or almost ready to fly (ARF) is a good place to start and learn building skills.
2. Engines increase in power exponentially so basically a C is twice as powerful as a B and a B is twice as powerful as an A. What motor is best depends on the rocket it is going to fly in. Most manufacturers like Estes state the recommended motor for the best performance. Once you get more into the hobby and build your skills you will probably start using a rocket simulator like Rocksim or OpenRocket to simulate your flights and help decide on the best motor to use.
3. One of the kits that a lot of beginners seem to start with are the Estes kits and a popular one is the Alpha III which can be found rather cheaply here:
https://www.acsupplyco.com/estes/estes_1427_alpha_3_launch_set.html
I'm sure others will chime in shortly with their suggestions. Welcome to the forum. You will learn a lot from the folks here.