Live Rock is wild rock, aquacultured LR is rock put into the ocean for a few years. That is a sweet deal on the tank BUT there is a major concern with it being a freshwater setup. You will want to ask if they have ever used copper based medication. Since it is still setup they should have on hand any medications used. If you can get the name of the meds you can usually look up the contents on-line. Cephs are sensative to copper and Jean (Portabello Aquarium, NZ) tells a story of a small amount of copper wiring being lethal and required extensive measures before the aquariums tank to be used for a ceph again. It is believed that copper medications imbed into the silicone and will leach enough to make the tank lethal for the life of the tank.
It would be a good idea to filter your rain water (and think twice about an RO/DI unit but you would need a pump in your cistern that provides at least 60 psi (higher is better and produces less waste). Pollens and other pollutants as well as mold and mildew are a problem as well as bacteria (drinking water cisterns must be treated with clorine). You might consider using the UV in the cistern rather than in the tank.
It would be a good idea to filter your rain water (and think twice about an RO/DI unit but you would need a pump in your cistern that provides at least 60 psi (higher is better and produces less waste). Pollens and other pollutants as well as mold and mildew are a problem as well as bacteria (drinking water cisterns must be treated with clorine). You might consider using the UV in the cistern rather than in the tank.