if you're wanting to do it the cheapest way you could get the pi 0 w, a standard wifi usb and a solar panel/battery combo.
Probably don't want to run directly off solar since it won't be operational over night, which means you should look for a battery pack that can be charged and discharged simultaneously. a quick search of reddit turned up this: https://www.reddit.com/r/raspberry_p...t_the/cu8oblg/
pi zero uses about .5 to 1 amp/hour which means you'd need about a 20,000mah battery pack at a minimum and probably a 15w solar panel which needs to be able to stand up to the weather wherever you are.
edit: oh and you'll probably want a waterproof container to put it all in. which means you'll need some sort of sealant to fill the hole you'll have to drill to get the usb power cord from the solar panel in.
EDIT2: and one of these if you're using the pi 0w and don't already have one: https://www.adafruit.com/product/2910