Translation:I would like a coffee without sugar, please.
This sentence is important for a place such as Guatemala where coffee comes with sugar already dumped in, so that it's quite sweet. If you don't like sweet coffee, you'll need to tell the waiter this sentence, to which (s)he will reply "para servirle." The situation with coffee in countries such as Guatemala is complicated. First timers usually think they'll get great coffee, but after having been there for a while or for a second time you become accustomed to the fact that most coffee in country is not very good, because they export the good coffee. A regular cup of Joe in Guatemala can be terrible, even upset the stomach. Exception: hotels and restaurants that cater to the wealthy 1% and tourists.
Yes, but in idiomatic English we don't say "I would want" but rather "I want." It has to do with English in this case, not Spanish. You can see how the two languages work differently here. You can say "Quiero" in Spanish but it is polite to use gustaría. "May I please have" is polite in US English, but there are regional variations.
In asking for something, a more polite form is usually used. The subjunctive used here is a bit like saying 'I was wanting a coffee...' - it's just very polite and less direct.
Quiero: To ask for something, it's unusual to just say 'I want' (toddlers excluded if course!) - It would sound rude in a lot of cultures.
Me gusto: You need the conditional here: 'Me gustaría' and in English the request requires the auxiliary verb 'would', otherwise you're just telling someone about your tastes rather than making a request.
You might find the discussion here useful: https://spanish.yabla.com/lesson-Very-Polite-Independent-Clauses-Using-Subjunctive-683
Quisiera is a polite way of saying "I would like." Here's an overview: https://www.thoughtco.com/making-polite-requests-3079221 You could go with Me gustaría. Me gusta means "I like," so doesn't work as you suggest.