You have a couple options and the lockpick is the easiest. Other options include tapping into the reverse camera input, and then making the MFD think it is in reverse. You get composite resolution but with the safety message overlayed on the screen.
It would not be wise to replace the MFD. There are too many functions tied into it, and even hidden diagnostic functionality.
There are inputs for the Prius that make it a VGA input which is really what you want. VGA is NTSC/PAL encoding agnostic.
SSD's are fine, so are harddrives. 3.5", 2.5" it doesn't matter. They all work just fine even with the road vibrations over time.
Also, you will need to do a battery upgrade. The stock 12v aux battery in the Prius has a very light load. All it does is prime the backup hydraulic brake pump and switch 3 relays in sequence to get the High Voltage (HV) battery online. Once the HV battery is online, it uses MG2 to spin up the ICE in lieu of a starter. So the battery has no "cranking" needs. Therefore it is very very tiny. All CarPC's draw power while sleeping or off, and will drain a "regular" car battery fast. It will destroy the Prius' aux battery. Get the elearnaid.com's Optima Yellowtop conversion kit as a necessity.
I suggest for now using the AUX input located in the centre console for audio input to the stock HU for amplication and through the speakers. That would be easiest.
Also I am sure you are already aware, but the false floor in the hatchback is a perfect place for a CarPC (and amp if wanted). Also it is right next to the battery. If you put the PC elsewhere, you MUST run a new wire to the battery because none of the 12v system wiring is up to snuff for a PC. Like I said, it has almost no current draw. You can literally "jump start" the prius with some AA batteries if you disconnect the dead SLA (otherwise it will try to charge the SLA from the AA's and won't work).