After the main component (SN), I think the antiemetic ranks 2nd in terms of importance. The other supplementary drugs are indeed useful, but essentially are not integral part of the basic SN poisoning mechanics inside the body. I personally only have access to meto, SN and a prescription antipsychotic drug Quetiapine which I plan on taking since I get debilitating anxiety amd panic attacks sometimes.
I'd say antiemetics are more important than benzos. You can still vomit while unconscious. It's implied in most guides that the ideal antiemetic of choice is something that is also a Dopamine Antsgonist or H2 inhibitor, although in some successful SN attempts, non H2 inhibiting antiemetics are used.