Whatever word count you need to get your point across. I think all of mine were at least 130 words (except the 100 word one). Granted, I am not in school, so I wrote a bit more for my activities.
They are relatively generic phrasings so you can take whatever path you want to answer questions. I chose to go with an outside my social comfort zone talking with some homeless folks, and then tied it back into one of my ECs. Everyone has a different life path, and your experiences will be different than anyone else's, so you have to decide if it works.
I briefly mentioned specialty but stated I am waiting for clerkships before choosing a path. I also mentioned how I want to go into academic medicine. I think, depending on how you phrase it, that mentioning rural medicine could work - urban may turn them off.
You can add letters after submitting primaries, and after submitting secondaries. Schools may not look at your app until they are in, or they may only look at the first 3 you sent in and ignore any you sent afterward. Each school is different.