If you are going to have an Arduino or other microcontrollers driving the speedo, and are capable programming it, you can do all the rest of your sensing/outputing with arduino(s). The fusion brain has a microcontroller that has an ADC, Digital outputs & has serial communication over USB to a windows based PC. The Arduino has a microcontroller with an ADC, Digital Outputs/Inputs (user defined) & user-defined logic & communication (serial, serial over USB etc). Think of the Fusion Brain as a specialized Arduino w/ a different IDE.
The Arduino is more low-level, but has a LOT more development/userbase. The FB has USB->windows interface software that is pretty, but I find it confusing (I'm apparently dense, so don't let that discourage you too much). I wouldn't put my only speedo/tach/fuel gauge into a 'CarPC' w/Nav/entertainment, a BSOD won't get you out of a ticket.... It sounds like you only want to electronic-ify a mechanical dash cluster, so I definitely wouldn't go with FB, as the only positive over arduino would be the windows UI on a screen.... You'd need a full-blown computer unit (one that can run windows for ALL the processing) as well. A bare ATMEL chip (what arduino is based off) is >10x less expensive than a fusion brain (but a few less input/outputs). I think the last ones I bought were ~$3.50 vs $50.... I got an arduino mini or nano w/USB for <$10, and I've seen cheaper on ebay...
I have a v3, v4 & 2x v6, but only the v4 ever gets used (and rarely). I've moved on to Rpi, beaglebone & arduino - more flexible, powerful & most of all, more active userbase.
Once you get the hang it, a voltage divider is extremely simple & you'll use it often. One resistor lets the lowered signal voltage go to the input & the other 'drains' the extra to ground. Wikipedia & plenty of other sites can explain it better than I can. You have more complicated things than that in your ambitions (steppers, opto-iso, etc). For us electronics dummies, blowing up $10, common parts is better than blowing up $50, specialized parts. I haven't seen the FB dev on here in months.
Edit to add: I didn't read the last 2 of your 'Aims', which complicates things greatly from a software standpoint. You could use FB or Arduino still, but depending on what software you want to use, a FB *MIGHT* be your magic bullet for hardware integration into front end software. I'd still suggest mission critical (fuel, speedo, headlights...) be separated from a windows consumer-grade PC. I'm not up to date with CarPC software/ Front Ends, you MIGHT be able to do all of this with a beaglebone (I'm sure you could, but you'd probably have to write a lot code, not sure if there's NAV or front ends that would run on BBB & you'd have the eggs in one basket problem)