Sigh. Okay, here we go. Go to the top of the page you are reading right now. You'll see a set of links that reads "MP3Car.com forums > Newbie". Click on Newbie. Look at the very first post that says "Sticky: How to get audio from your computer to your speakers". Click on that and you'll get a nice discussion on how to do what you're asking. If you can't find it, here's the link: How to get audio from your computer to your speakers.
It's not necessarily cheaper and easier. When the battery goes low, it cuts the power to your computer. That may or may not scramble your hard drive requiring you to reformat and reload. Go back to the "MP3Car.com forums > Newbie" section again. Look at the post directly below the last one titled "Sticky: Frequently Asked Questions". It answers some of your power questions and discusses the differences between the various power options. There's other stuff in there too. Here's the link, in case you can't find it: Frequently Asked QuestionsI say inverter becuase it's just cheaper, easier, and if batt goes low for some reason it turns off it's self.
You want to go cheap. I understand that. We all do. Just remember there's usually a tradeoff between cheap and "degree of difficulty". These forums have tremendous amounts of information on nearly any aspect of car computing that you can think of. Get aquainted with the search function -it's located on the title bar up there. Try it out - type in "carpc cost" and you'll get lots of results, some of which discuss what others here have spent on their PC setups.
You can also use it to search on terms "Best GPS" and the next question you'll have "Best GPS software".
Welcome and have fun.