Where to stay in San Diego, given certain needs?

I am not familiar with San Diego. I was there briefly many years ago, but don't really remember much. I will only be there for a couple of days. The areas I intend to be in are Balboa Park/zoo, the Gaslamp, and a beach. I am flexible on the beach. I'm thinking maybe Mission though. I seem to remember liking Mission the best when I was there in the 80's. I would like to avoid renting a car. I would like to get around mostly by bus, with one or two cabs, if necessary. I don't have a lot of money, but I'm willing to extend myself to make sure I have a good trip. I was looking at the Marriott in the Gaslamp area, but then I thought maybe it would be better to stay by the beach or the Balboa area, since I might only be needing to be in the Gaslamp area for one evening. I have some neurological problems, so I definitely require peace and quiet (as in, not staying in a place with paper-thin walls), and also a very comfortable bed. Specific suggestions for hotels are great, but I'm primarily looking for guidance as to what part of town would be best in order for me to accomplish where I want to go and being able to take the bus to get to places. Also, I want to be able to walk out of the hotel and be near restaurants and places to walk around in general, not just off of some highway exit. And, I would like to feel that I'll be safe, both in the hotel and walking around. Thanks much for your recommendations!