1. How hard is it to change the gears out on my own?
2. Assuming everything is good inside, will I need to buy any kits as well or just the ring and pinion? Need to get this ordered soon.
1. Not easy. You need special tools that most amateur mechanics don't have and/or don't know how to use. The gears have to set up to mesh properly, otherwise they will be noisy and/or wear out quickly.
2. Yes, no matter what you'll need an installation kit.