You have much to learn.
Star here: http://www.mp3car.com/vbulletin/advfaq.php
HDD life in a car is not an issue.
The graphics card is overpowered and should be replaced with something that uses less power. Onboard video is ideal in that regard.
You can keep it right about your $300 budget by keeping what you have and getting a DC-DC power supply and touchscreen.
Search the forums for information about e-Machines. There may be issues to overcome, depending on model.