Having been down the road you are attempting to travel several times, I'm going to suggest that you don't go with a Mac Mini.
I'm not trying to argue with your motivations for doing this, and I'm not trying to pee in your cheerios, but this is going to be a much more expensive and time consuming endeavor than you think, and in the end you might not be happy with the result. This is just friendly advice, do with it what you want.
I had two similar setups, a mac mini with a lilliput VGA touchscreen, and a 1st gen Apple TV on the factory screen in my Acura TL.
The mac mini setup was a nightmare. First, you can't just use a cheap inverter to convert your cars DC to the AC for the mini. You are going to get lots of noise in your audio (connected to your stereo) and most likely video signal because the inverter won't do a good job of filtering out the crap in your cars electrical system and it doesn't provide a very good ground, or at least one that is consistent with the ground of your cars electrical system. So your choice is to get a really expensive inverter designed to be clean, or there are companies that sell a replacement DC power supply for the mini. do a search on the forums at mp3car.com for that info, but again more expense.
Then once you have the power supply figured out, you need to manage it. You can't just shut the mini off with your key, over time the OS is going to get screwed by missing and corrupted files. You can't leave it on because its going to drain your battery faster than you think. The mac mini car power supply being sold isn't 100% reliable, so you need to plan for a clean boot-up/shutdown everytime you get in the car. My older mac mini took about a minute to boot fully and about 45 seconds to shut down. Doesn't sound like much, but it pretty much makes it useless for a short trip. If you are running late for school/work/a date/whatever do you really have 2 extra minutes to sit in your car? Next time you drive somewhere, sit in the car an extra minute or two after you shut everything off. It will seem like a lifetime. People have come up with solutions at mp3car.com and they all have a downside. Plus, you need a way to access the power switch, or at least rewire the existing one to where its convenient from the drivers seat. Its going to lock up at some point (due to a failing HD or corrupt OS) and you are going to need to do a forced shutdown/restart. Also, say you are watching a movie or listening to music on the booted mac mini in the ACC position, then go to start the car. The drain on the battery from the starter will most likely cause a voltage drop low enough to shut down the mac mini.
Then, how are you going to sync your material, or at least new material. Wi-Fi? Portable HD? String an ethernet cable to your car at night? It can be such a PITA that I had nearly the same content a year later. Think about what you have to do to import content into iTunes at your desk, then transfer that thought to the drivers seat of a car that needs to be running (or has a limited time on the battery). You could bring the mini in at night, but are you really going to do that?
Then budget for hard drives, because you are going to kill them with the heat/cold/humidity/condensation. And its going to happen at some shi*ty time like when you are about to leave on a road trip or when you are showing off your system to your friends. You are planning a massive amount of storage, think about how long its going to take you to restore it all onto a new drive. You could go with SSD, but then you are getting into $$ for the amount of storage you want.
Then think about the display. Most are 800x480 native. Set your desktop to that and see how damn unusable a modern mac is at that resolution. Your menubar across the top will not fit. You won't be able to use iTunes. If you go higher res, the fonts are going to be tiny unless you have a huge monitor in your dash. You sure as hell can't read it at 65mph. I at least had the advantage of Front Row, but that is obsolete in later versions of OSX.
That pretty much sums up my experience with the mac mini. I switched to an appleTV1 because the interface was more usable for a car environment. I bought and installed a custom 'pico' power supply to run it directly on 12Volts. Same issues with power/killing hard drives/spending more time making it work than using it.
Then, the most fun part. You leave it on accidentally, or whatever power off solution you come up with fails. You come out to a dead battery the next morning. Do that enough times and you kill a cell in the battery then you need one of those too.
Say none of that scares you off, you are looking for a challenge and you are saying F U brucewayne I'll do what I want.
Here's the thing. You will never, ever, ever, ever, ever use terrabytes of content in your car unless you are a passenger on a cross country road trip. You are listening to a guy that spent many wasted years converting 5+TB of music, TV shows, and movies for iTunes and apple TVs.
First, think about the user interface, even a best case scenario in that you are using Front Row. If you have tens of thousands of songs, how long do you think it will take you to get from Afroman to Weezer? Then find the right song?
TV shows and Movies, if you have any sense it will be cool for about 5 minutes then you will say to your self "holy ****, i'm going to be in an accident if i keep doing this" and you'll never watch another show again. Except in the rare case that you are sitting in your car while your girl/guy is shopping or something, in which case I'd bet a dollar you will be on your phone checking messages or surfing the net.
"But what about my passenger, brucewayne, won't I be hella cool if I can play Doctor Who on the way to a football game?" No, because most people get motion sickness staring at 8" screen in a moving car. So your date either blows chunks, or says turn it off, or even better says "isn't this unsafe and illegal to be driving with a movie playing in the dash?"
Again, this is coming from someone that spent a lot of money to do what you want to do, and in the end I ripped it all out and put an iPad mini in my new accord. It has enough storage for actual use in a car, has OBDII connectivity via dashcommand and the Gopoint BT OBD adapter, has LTE for internet, pandora, radio, and Siri (i.e. i can say 'take me to Chik Fil A, or play Metallica), has GPS, and is portable enough to bring into the house at night to sync content. I can stream from my home network via the AirVideo app or slingbox. It will never kill my battery, and because I am using a digital audio connection (via USB) i never need to worry about noise or crappy grounds. And if I'm waiting for my wife in a parking lot somewhere, I can pick it up and read a book or surf the net. I would do this setup again in a heartbeat
"but I have a sweet LCD being installed in my dash right now".
$99 for an appleTV3. Find a 12VDC to 3.4VDC power supply, open up the ATV3, remove the AC power supply and modify it to run on the 3.4VDC power supply. ifixit has the info you'll need.
$35 or so for a monoprice.com adapter to convert the ATV3 HDMI to whatever signal the LCD has for an input. run the audio to your stereo.
Turn on WiFi sharing on your iPhone. Connect the ATV to that network. Use Airplay streaming for content from your phone. As a benefit, mirror the display and see your phones screen on the car's LCD.
The above will be a hell of a lot easier to implement. The ATV will use a tiny fraction of power compared to a mac mini. You can turn it on and off at will, just like you would an iPod. It boots up much faster and is ready to go. If you go for an ATV2 you could jailbreak it and do all sorts of stuff. You probably keep the content you are most likely to use on your phone anyway, so it will always be current.
Again, I understand why you want to do this. I just implore you, for your sake, don't. You will spend a lot of time, money, energy into something that won't satisfy you. A tablet just makes so much more sense.
