50s- low 60s is the ideal temperature. The only feasible way to achieve this is with a chiller (I suppose you could use several fans, but I'd rather not comprehend the top-off
). They need a super sterile environment: sump, UV, O3, and a controller would be nice. Ambient light is all that is needed and all that is best, really. You can blind them with a lot of light. Use a small amount of live rock (just enough so the Nautilus can have somewhere to hide, if he feels necessary). If you really want to mimic the natural environment, use a controller to cool the tank to the low-mid 50s during the day and raise it to upper 50s low 60s at night. Salinity should be the same as Natural Sea Water at 1.026. A standard 75 gallon tank is recommended, so that the Nautilus has less of a chance at running into the sides, damaging it's flesh or shell.
If you are going to buy one let us know before you get it, certain things need to be done to ensure there is no air stuck inside the shell and that the animal has a successful acclimation. They are very expensive to keep, and not the most interesting, but it would be cool to have one.