Maybe the standalone ISO with just the arduino and the MC33290 still makes sense, it covers a lot of cars and is very cheap to build. The ELM version does not need the freescale, take a look at the PDF page 56
http://www.elmelectronics.com/DSheets/ELM327DS.pdf
I got the MCP2551 free from Microchip as a sample, and I will order the ELM327. What's nice with the ELM327 is its cover of
all the protocol. Also the reference design is available and if you just want CAN for instance you just have to buy/solder the CAN part and put some pins to ground/5V.
see page 58 of the PDF for an example for VPW.
See ¶ "Where can I buy a circuit board for my ELM IC?" of this url, almost at the end, for schematic, gerber, etc
ELM Electronics - Chip Help
Also OBDuino and MPGuino share a lot of code. LCD and buttons are the same (I optimized some LCD functions). In OBDuino there is a level of abstraction for the protocol layer. There is a generic get_pid() function that return the result, inside there is a call to iso_send_cmd() that can be changed by a parameter and call vpw_send_cmd() or elm_send_cmd() for instance.
Also if we have an hybrid setup that can work with the MC33290 and/or the ELM327 and the code is less than 14k, why not?