In case of the indefinite article here, I admit that it is possible. If I'm not mistaken, this will be a case of ellipsis. Where do you think the ellipsis is?
ButWhy we can't omit the article here? Don't try to start a sentence with "but".
I don't know if there's a rule or principle but when you say you need "extra 50 computers", an article is needed before "extra", as #4 says. You need "an" if you're talking in general and "the" if you're referring to 50 computers that you'd already mentioned or that the other person knows about.
For it to be an item on the agenda, it must have been put forward by at least one member of the committee. Items don't just magically appear on an agenda for a meeting.