Duolingo isn't testing you in your comprehension of Irish, or your English vocabulary, it's teaching you a basic but functional vocabulary in Irish. In that basic vocabulary, fonn is taught as desire, and rendered as "want" in this sentence, because it's a better idiomatic sentence in English.
The contributors didn't go through a thesaurus and add every possible translation to every exercise. You can request that they add alternative translations, but really, is it worth their or your effort in this case?
I think also that the meaning of "want" in English (at least in the US) has drifted from the meaning "need" and "lack of" over toward the idea of "desire". We read in literature but rarely say "they are in want of x" when we can more commonly say "they are in need of x". The sentence "Tá ríomhaire uaim" expresses that the computer is "wanted" not because of desire so much as because it is "needed" (for not being here, i.e. "at me" or "have"). Tá fonn orm ríomhaire níos fearr ach níl ríomhaire uaim. "I desire a better computer but I don't need a computer" is more understandable than "I want a better computer but I don't want a computer." Tá súil agam go raibh sé cabhrach.
You can use a as a preposition between a noun and a verbal noun (leabhar a léamh - "to read a book", litir a sheoladh - "to send a book"). But you can't have a preposition after a prepositional pronoun, as far as I know, so I don't think you would use a in this case. (In fact, I'm not sure that I can think of a way to use dul in that kind of construction).