The Parts

In my experience the best way to build something is actually to find something that does almost what you want, and then modify it. It is usually both cheaper and easier than to build it from scratch. In this case I found two brilliant parts: One windshield cleaner tank with two integrated pumps for 149 SEK (€15) and a remote controlled LED spotlight which Nethome can control for 129 SEK (€12).

The tank with pumps, I can use as it is. I tested it, and it works fine, but it draws a LOT of current: 2-3 A when operating at 7 Volts. It is designed for 12 V, but I don’t need so high pressure and want to have fewer batteries (and lower power consumption). The tank holds 2,5 liters which is a bit small, but I can connect it to a bit larger tank via the unused pump connection when I need to be away for longer periods.
The spotlight uses the 433MHz band with the NEXA-protocol  for the remote control. I will just use the receiver from it, so I have to take it apart and try to find the on/off signal which I will use.