For the MPG how about a pair of these. Flow Sensor One on the fuel intake and one on the return line. Then you just take the difference and that is fuel used. Use either gps or something else to figure out distance and voila. MPG.

As for the old computers I had one and the instructions said that it needed some additional parts to possibly work with diesels. I don't have it anymore cause it was one of those things I bought NIB for $2 at auction and sold for $150 on ebay.