Yes, this forum has lots of answers for your type questions.
If you have never built a computer before...then I would suggest building one in your house...make sure it works to your liking...then stuff it in the car.
I assume you will stick it under the hood (over gas tank)...seems to me the 74 beetles were a bit tight underhood...with AC and things...and the fancy dash.
Power will be your big decision. So search for power supplies...look at folks installs, and read their specs.
Sound output might be important to you...sound card sure...and some way to get it to the HU.