If *I* were to play around with an obd version it will be elm based, and use the less expensive single protocol chips because:
1. they are less expensive
2. they are less complicated to hookup (only care about a couple obd pins, not all of them.
3. I already have a 323
Here are some schematics:
http://www.scantool.net/schematics.html
[looking at the 323 elmiso schematic] if it is connected to a duino, you can skip the rs232 stuff on the lower right , just use ttl to the duino tx/rx ports@9600, and all the blinky LEDs in the upper right and the voltage regulator circuit. So you'd be left with 8 resistors, 3 transistors, 1 crystal, 2 capacitors, 1 diode, and of course the elm323 and an obd plug, just for iso protocol.