After discussing this at the curry I'm really pleased that you've decided to give it more thought.
I think for something to work as an actual dash replacement then it would have to look 'really' good in terms of the quality of display and the fitment / surround that goes with it.
I personally think that something designed to be DIN sized (not necessarily the whole thing - just so the box fitted there) would be the ideal solution.
To me the critical aspects in terms of it being a useful driving tool are:
- Monitoring all the sensors that it is connected too.
- Configurable alarms on all sensors both high and low values
- The parameter that has caused the alarm to be displayed on the screen
- A driver to an external output so that the device can trigger a bright LED/loud buzzer etc
From all my research into the area (and I have done a lot) trying to find a solution for my car, I can report that there are many many options out there however they all fall down on one more more aspects.
There are very good systems that meet all my criteria but they are well over 1k. The cheaper systems appear on the face of things to be good value but then they typically all fail at one or other hurdles such as not displaying the alarmed parameter on the dash or requiring resets to look at different readings, through to the way in which alarms are interpreted and requiring specific and unusual sensors.
I think from the above you are proposing to use the standard sensors, my feeling is this might be a bit ambitious and there are several important areas that the standard setup doesn't monitor like EGT, Fuel Pressure, Oil Temperature and Oil Pressure. Given that plumbing most of these sensors in is not that difficult I would be more in favor of a display using additional sensors of a standard type (bosch temp sensors & wideband, k type thermocouples etc)
Just my thoughts.
In terms of legality etc - for dash replacement there is a product called Digidash which is very popular in the kitcar world which provides an all in one solution in terms of the statutory instrumentation and some extra sensors. I don't think it can do stuff like EGT and widebands but might be worth looking at for ideas if you want to go down the dash replacement route.