Most guitarist have a big beef with acoustic guitar pickup systems, because they just don't sound as good as what an acoustic guitar sounds like on a microphone in the studio. In a typical live rock band situation, the biggest problem using a mic on an acoustic guitar is since an acoustic guitar...
I always tell people to buy one mics to start a studio with. A Shure SM-57. It's a great mic for guitar amps and vocals. After that, it's a matter of finding a condenser, which fits your budget.