I've just started investigating OBD because like you I have multiple cars
for which I'd like to find a single solution.  I recommend you visit
www.obd-2.com and particularly http://www.obd-2.com/dlcnet.html which will
allow you to enter your vehicle descriptions and they will provide a quote
for the least cost solution.  Unfortunately for you the first model year
they cover is 1994.  I plugged the last 2 cars from your list into their
page and it told me that the cars use different interfaces (there are 4
different OBD-II interfaces) so you need either two different pieces of
hardware, i.e. an ISO and a VPW interface, or perhaps their new TriCan would
be a better choice (more expensive) because it handles all 4 OBD-II
interfaces.

This is just a place to start.  You'll get lots of hits for pages describing
other systems if you do a little googling.

BTW, I'd like to hear recommendations from any readers of this thread who
have experience or knowledge in the use of open source software for OBD.  I
need to get ATF temperature from a 96 VW Golf and I think that I probably
need to make my own modifications in software to get that value.