Well as far as I recall the ODB2 sockets are supposed to be a standardised thing these days, one assumes similary the hardware to read an ODB2 socket will be of the same design regardless of vehicle. I think where the main complications lie is with software and method of communication. I have had VWs for years now and my old mk3 golf had ODB1 socket, which is the same design as ODB2 but is much more limited, for just a few quid on ebay you can get VAGCOM cables, now I know this is for VAG group cars, but do some research and you may find that it will interfact with your vehicle.
The problem comes actually talking to the ECU.
When you install the VAG come cable, it actually uses the same FTDI chip as interfacing with the arduino, at least from what I have seen so far that seems the case. so a USB to serial coms device is the common FTDI chip, as I previously mentioned what maybe more of a problem is actually communicating with the ECU itself.
An arduino can be built in basic form fairly cheaply and I bought a prebuilt one for about £14 to experiment with (but I think you can build one for about £7 for a basic) there are some tutorials online that show you how to build an ODB data output reader, might be worth reading through one of those to see if its possible. I know in 2003 VW changed the standards of their ODB2 ports which meant as a consequence you couldnt use the FTDI based interface with the new cars anymore, it maybe the same sort of problem with Smart/merc/bmw that they all went the same way.