I'd love to build a HUD for my Subaru BRZ like this, but using some kind of HUD. Even just as a speedometer. But I'm not really able to solder/do much wiring since I have hand tremors. It'd be sweet, though.
I did a lot of soldering as a microwave tech, and used a setup like that just for convenience. I have to have a magnifying glass on a similar flex arm because my eyes are so bad. I suppose there are still some hard parts, like trying to use braid to remove solder, but I bet you can do it.
I have a 1991 Mitsubishi Galant VR4 that has a place in my heart that your Subaru seems to have. Cool family compact with 4WD and 500 oversized, at the wheels, turbo HP.
I wonder if you could use something like this [0] and feed info from the pi over OBD using some sort of OBDII breakout board or OBDII <-> USB device.