Guitar, keyboards, and voice should be easy enough in MS, backing tracks are a different matter. For the Guitar and Voice, you'll need a small USB interface, which range from about $100 to $400 depending upon quality. PreSonus, Focusrite, M-Audio, MOTU, Apogee, Lexicon (and others) make good interfaces that should meet your needs, and interface with any audio host (like MS). Then you simply have a patch that contains two "Audio Channel Strips", one for voice and one for guitar, with any effects you need for either. Any time you need different settings, make a new patch with the necessary effects and parameters.
I just learned that MS can do basic background tracks. Here's a YouTube video that shows how: https://www.youtube.com/watch?v=PUDFoEm1FPE. Now, you're basically talking about a wav or mp3 that plays from start to finish... no looping or other things, for anything more involved you'll want to use something like Ableton Live, which is what I use for triggering live drum loops and stuff. But if you've already bounced down audio backing tracks, MS will be fine!