Good Day Carusoam
Initially I just wante d to buy the display and the extra sensors, but when I contacted JPI they said that they are normally sold as a complete set because this is a TSO Primary engine instrument replacement. Therefor I had to buy the complete set. The good news is that many of the probes and sensors (EGT, CHT, FF transducer etc) were the same part number as the ones I already had installed so I did not have to replace those. Some of the sensors were new and not part of the JPI 700 setup so I had to have those installed (RPM sensor, MP, FP, OAT). The factory CHT and Oil temp were removed and so was my JPI 700 oil temp sensor. The part number for the 700 oil temp was different than the 900 oil temp probe so it was swapped out. Although I am not the legal authority on this, I believe the bottom line is that as long as the JPI sensors you already have installed are the same part numbers as required by the JPI 900 primary system I think they can be reused.
With respect to the the fuel gauges, you can buy new fuel senders if you want, but whether you put in new senders or not in your tanks they will have to calibrated using the sender resistance as measures by the JPI 900. Hence there is really no need to change out your senders as long as they are still working. After calibration of the old senders to the JPI 900 my fuel gauges are reading within 1 gallon of what I measure when I dip my tanks (my home made calibrated dip stick may be in error as well with such a small difference). Over time I will collect more data (using the fuel flow computer, dipping the tanks and the fuel gauge read outs) and adjust the calibration slightly should I feel the need.
Hope that is helpful and at least understandable
Cheers
Edward