Hello fellow Scion Owner!
I am currently installing a carputer in my Scion.... in fact I am getting ready to start the wiring as we speak.
There is plenty of room in your car for computer things.
I chose the 7" screen so I don't have to modify the dash, whereas the 8" would require some cutting and it is a 4:3 screen vs. the widescreen.
As for your question, it would serve you best to look at some of the newbie threads and specifically the Power supply threads, they will give you a lot of info.
You have a couple of choices on power the inverter or the DC-DC power supply. THe inverter alone would be quick and easy, but not real practical without some sort of controller thing, that I can't seem to remember the name of...
The DC-DC power is my choice. I am running a 90W Opus, but I haven't had a chance to try it out.
The big question will be what type of Computer you are running. You would need to give some specifics about it. ATX, mini-atx, ITX, etc.... CPU, memory..... then do some searches for these items that relate to your particular set-up. Since no two systems area like, there is'nt a definitive answer to your question without more information
Good Luck, if you have any spcific questions, feel free to ask me.