My room is 9.5 ft x 11.5 ft

My table is rectangular 5.5 ft.

You would want enough room behind the chair that if someone was seated, you could pass behind them without difficulty. Doing a mock up will give you an idea of how much depth is comfortable.

I don't think it is a requirement to have a buffet, I think that is a personal preference for what your needs are. I don't have one in mine. I do have bookcases in my DR...it is sort of a diningroom library. I love it, it suits my needs for having a tiny home and making the best use of space for me.

Here is a link that might be useful: Info on distance for dining chairs to wall etc..

If you have a small room (and/or want a library room) then definitely skip the buffet. Bookcases are shallower and you can leave a buffet-level tall shelf for serving if you really need it.

You really don't want to be saddled with a buffet that doesn't quite fit. I do store a lot of stuff in mine but I wish I had a place for it somewhere else...

Definitely no requirement! You don't even have to have any other piece of furniture - you could warm things up with large artwork or a large mirror.

But I like the idea of bookcases. Perhaps finish a shorter wall entirely in floor to ceiling bookcases. (You can buy stock bookcases from unfinished furniture places (or even Ikea, if you don't mind it not being real wood) and pretty inexpensively do a built-in look by trimming them out with stock trim.) Or, center one large bookcase on a wall. It's also an interesting idea to do shorter cases and leave a shelf on top for serving.