Question about solar panels

Totally unrelated to anything beer, but there's tons of knowledge here so I figure someone definitely knows.

We have an awesome greenhouse set up, but we need a way to run a fan in it so the plants don't get cooked in the summer. Rather than dig a trench and run cable out 75 feet from the house, I was thinking of just a small solar panel.

Just a few questions. We would probably get a 12V fan so as to not have to mess with an inverter.
As the sun output varies, the voltage of the solar panel would remain constant, and the amperage would vary, right? Would this hurt the fan motor? Would we have to over-provision the panel to ensure the fan running all day? Would it hurt anything to just have the fan turn itself off, so to speak, by the sun setting. A battery backup is unneccesary, as well as cost prohibitive.

Google solar attic fan and that will give some great ideas and you can get off the rack if you want
Basically runs only when reaches certain temp and needs no batteries been looking at them myself