there's definitely been lots of good advice here. personally i'm in the garbage in=garbage out camp. if you're doing mostly vocals and ac guitar, you might be able to get away with some more makeshift acoustic control (e.g. blankets, sleeping bags, duvets, etc. positioned strategically). things like mics, preamps, ad converters, actually change how the sound will translate to your hard drive. monitors do not and ultimately most of your listening audience will generally listen on hyped speakers (in their car or personal stereo systems) or on earbuds anyway, which is why many studios will have monitors like ns10's or auratones (or avantones now) next to their adams, dynaudios, etc.
short answer-- imho, all of the things mentioned are important, but i'd get a mic. ymmv