then your best bet is the search, and read as much as possible.
expect to make mistakes-- they are the things you will learn the most from, just try to not make very many
1.prebuilt units: i am not a fan of them, primarily because of the problems they can lead to-- if you don't have the time to build a pc, this means that a car pc in general is a bad option--you probably won't have time for the rest of the project either. if you are great with software, but horrible with putting all the hardware together, then they are good options. for someone new to computers, i highly recommend building your own-- this is primarily so that you can learn what is needed, why it is needed, and how everything goes together, so when something breaks, it becomes easier to repair..
2. used pc's are hit-or-miss. just like playing the lottery-- some are a great deal, others aren't..
3. diy pc's: this is the route that i really support. imo, it makes it easier to trouble shoot things i have put together then to figure out someone else's mess.. and as stated above, if you are new to computers, or installing them in cars, this is a great way to introduce yourself to all the parts, and get to know how they all work together.