If you already have a HDMI monitor like a newer TV or computer monitor, you need only keyboard, mouse, power supply and a micro SD card, 8G or larger. With those things, it will act as a stand alone computer, albeit a slow one, depending on model. You don't say what you want to do with the tank so that is as much as I can tell you. If you need sensors, you may have to adapt existing code to your needs and Python seems to be the language of choice for most of them but there is quite a bit of code out there, already written. The sensors themselves are separate purchases.
Sent by WH7QQ from an RPi3/Raspbian/pixel desktop.