Hey, I've wanted to do the same thing for a couple of years now, I just haven't had the spare time and spare cash at the same time!

I think for the plc hardware I would probably go with either the 205 series with a 260 CPU or the 06 series, all depending on the number of I/O you will need. For the HMI I would try to use the biggest C-more that I could get in the dash. The speed sensing would be simple, a prox sensor on the crank, and one on the driveline with a high speed counter and you've got it. For oil pressure you install a 4-20ma pressure transducer and an analog input card, for temperature I would go with a RTD sensor for the coolant and transmission,with a RTD input card, and you could also add intake and exhaust temps if you wanted. For the voltage you would need a voltage to current transducer, and for the amperage you would need a current trasducer along with a shunt, both of these can use the same analog input card as the oil pressure. For the transducers, check out ohiosemitronics.com. I am not sure about changing the timing, it would all depend on the type of ignition system that you plan on using. Everything else should be discrete I/O. Hope this helps, and please let me know how things go, Sounds like a lot of fun

Jason