First that I can suggest is the wonderful F.A.Q's First of all. As for your question though.... it comes down to 1 thing.... what do you want? Do you want to be able to take your laptop in your house or just out of the car and use it on demand? Because if so, if you already do that then use the laptop w/ a port replicator or docking station.
But on the other hand if you are going to be using the system exclusively for your CarPC needs then certainly build your own. You have more options that way, you can customize things alot more!! I built mine