If your sole motivation for a CarPC is to save money, don't do it. It will only end in frustration.
If you want it because you're a geek, and saving money is just a bonus, then
I have a 2001 ex coupe which is exactly the same as yours (7th gen). I got my first system together for real cheap. We are lucky because once you take out the OEM HU, there is a space that fits an EBY701 Lilliput ripoff screen like the 7" Ex-Audio screen perfectly, and a Metra kit can go around it to make it look like a bought radio.