Building a touring/4x4 vehicle data logger — what would be the best da

Everything you've listed can be done using an OBD2 reader and the Torque Pro app or Apple equivalent. Wrt parameters, coolant and transmission temps for on-the-go visuals, then other parameters such as intake air temperature, rail pressure for problems but proper interpretation of these is level above my expertise.

Hairy.
If you monitor the voltage and it is insufficient, then WHAT!
How would a monitor be able to show a BATTERY struggling to charge? If volts and amp supply is sufficient, it will charge, and you still won't know if it has the capacity to start the engine later. SO what does struggling to charge mean?
With the starter motor failure, no meter will tell you that! How could you predict the burning of contacts to stop the starter operating??? or a MECHANICAL STARTER FAILURE???

I use the fuel gauge to see the usage and conditions vary that too!
I made a system to indicate if head temp got too hot, ie, sounds an ALARM. It is the best way!
Apart from voltage, maybe, and water temp, you have no control over most of it anyway so not much use storing the data. WHO is going to interpret it later? And what are you going to do about it?

I think your way over thinking it. Get an Ultragauge or Scangauge and they can tell you everything you need to knwo. I,ve had batteries that have tested well and a week later have dropped a cell and become useless..no tool will predict this. I,ve been monitoring my vehicles performance with an Ultragauge for 6 years. It works brilliantly and preset alarms let me know of pending issues. Battery inputs will vary with revs of the engine and battery output usage...these can be all over the place and knowing the figures on a trip is of no value. I fail to see the value in knowing that my vehicle was putting out 13.5 volts at 100kph at gps co=ord.Lat 1234 long 12324.. The answer to your last question is simple...get an Ultragauge or Scangauge and this will monitor everything you need to keep an eye on your vehicles operation whilst traveling. I monitor gps speed, intake temp, engine temp, transmission temp, volts, boost and if you want you can do fuel usage , trip time etc, etc......well worth the money and not expensive.

There was a couple of ODB units that allowed you to select and log ODB data, one such unit was the "DashDyno" from unit from Auterra. It had a graphical display, alarms and external inputs, allowed reset of fault codes and had trip computers etc and was well in advance of the scangauge units of the time. There didn't appear to be much demand though from the general public market for data loggers. I used a DashDyno for many years and only repalced it recently as the company was no longer updating codes and I wanted to monitor my trans temperatures. In the many years I had the unit I never used the logging capabilty apart from the trip meters.

A quick check of the net gives a few hits for ODB2 data loggers which aren't expensive so if your considering to develope your own to market I would suggest you research what's already out there.