SM58 is probably your best bet for vocals at this point. SM57/58 aren't great for recording acoustic guitar, they yield great tone, but need a lot of gain to function and I'm not sure if your device has it. Condenser would be better but since you said your interface doesn't have phantom power that's out. Hmm, maybe an Audix i5? it's similar to an SM57 in response except that it needs a few dB less gain to function. It thins out a little bit of mid-mud too so it'll be great if your acoustic is a bit on the bassy side. Ooh, here's an idea. How about a Shure Beta series mic, in my experience those mics need significantly less gain then the SM series to get signal from. That's a good solution in my opinion.