"I'm going to go for" would be best. The others sound oddly formal. He could also say "I'm going to get the espresso."

All are correct. "Going to go for" is informal, "choose" is neutral. "Select" is formal and unnatural in this context. Also it would be more natural to use the contraction "I'm" instead of "I am:" "I'm going to go for." All of them suggest that Ken was having trouble choosing and asked Tom for advice. It would be natural in a context like this: Ken: "What should I have? Café americano or espresso? The espresso is awfully expensive. Tom: "The espresso is really good here." Ken: "Ok, I'm going to go for the espresso." If there isn't any discussion of alternatives, Ken has no reason to say that he's making a choice, and would probably just say "I'll have the espresso."

The question should not be "which is better?" All are perfect. The question should be which word best corresponds to Ken's personality. The way people speak reveals who they are. Without knowing Ken, it is impossible for me to advise him what to say. If Ken is a precise person who likes to be decisive, he could say "select". "Choose" has more of a feeling of randomness than "select". If Ken is a person who has a bit of difficulty making up his mind, this is the word he should use. If Ken likes being adventuresome, "go for" would be a great choice.

They are all rather odd. The choice is implied by the context and doesn’t have to be repeated. You can simply say: I’m going to have … I’ll have …