From what I have seen, I think that a kit from 14point7.com is a good option. They are reasonably priced, and the owner has posted some interesting tear-downs of competing products (including the LC-1) to compare them to his own stuff, and his stuff appears to be well designed. The Spartan Lambda controllers are the simplest option, but I am not 100% sure that the narrow band simulation voltage levels will be compatible with the Motronic ECU. You could go with the SLC-PP2 unit, which is a bit overkill, but it has programmable output voltages so you can make sure that they are in the 100-900mV range. I have asked the owner about the voltage levels on the Spartan NB output and am waiting for a response.