Shad3s,
On the full spec ones, you will have a monitor display either 4" or 7" controlling radio, navigation, tv, on board computer(obc) display, aux vent control for in cabin, dsp etc... Other accessories are memory seats, rear screen, memory side mirror.
Engine wise you better get a mechanic and hook up to see the history of service and part replaced (some electronic parts will be recorded such as fuel pump, actuator etc). One major point to lookout for is the cooling system. There are few reports on Jap spec E39 with hairline crack on cylinder 1 causing coolant loss.
M52 engines run on cable and M54 on wire. You can find out the spec using the VIN and TIS.