This is a pretty big project. Yes, get an arduino MEGA 2560 R3 for this job. For water level sensing, you can also mount a sonic ranger on the lid of your tank cover to let the sonic ranger face down. Water will reflect is sonic pulse and you get distance from water surface to the tank lid. If someone removes the lid then the reading will be like 6 meters. If distance is an issue (15 m), do it wirelessly then. I don't know your budget, if you have enough money, have a tank unit with arduino and xbee radio for wireless communication with an apartment unit with another arduino and xbee radio. This way the wires are gone. You do need a solar charger and battery for the tank unit, if there is no power outlet there.
I use a lot of xbee products to build data loggers. They are nice.