What can cause a Garbage Disposal to trip the breaker besides blockage?
Well, I have to admit I am not a Garbage Disposal Expert, but if the key turns freely and it is tripping the breaker, then I would think it is time to go shopping.

Is it easy to install? That's pretty subjective and depends on your skills. But, in general, they are not difficult for someone who is use to doing DIY stuff around the house, especially if you are replacing it with the same or very similar model. Find a brand that you are interested in (In-Sink-Erator is pretty common, but by no means the only one), then go out to their web site and download the instructions and evaluate it for yourself.

One word of caution if you try to do it yourself. Do you have a dishwasher? If so, you will see a hose from the dishwasher going into the side of the disposal. On a new disposal the assumption is you don't have a dishwasher and this tube is sealed! If have a dishwasher and you don't open it your dishwasher will back up. Also, after you knock out the seal, remember to remove it from the inside of the disposal since it is hard plastic.