What electric parts do I need for setup the Dryer?
-> You can buy this fan in many computer-shops as spare-part. And instead of a Solar-Panel you can take a cheap Power-supply, 6-12 V Output. If you have one with a switchable Output-Voltage you can adjust the speed of the fan (and with that the Temperature). It helps having a cheap Temperature-Display in the dryer for to check the Temperature. But we have also a small electronic Temperature-regulator for to have more accurate-Temperatures. 

The back paint shhould be compatible with the food?
-> Not really because you do not fill food in this black area. 

How thick should be the plastic?
-> The plastic thickness should be that it does not get tear apart to easy and should have some stability. UV-Resistent means, that it does not get spoiled to soon by sun, because sun is destroying plastic more or less fast. Anyway don’t worry, the biggest problem will be to get plastic-foil of that size. Of course it has to be transparent for the sun to go in. If this size is not available may be you can glue several foils together. Or you leave the part above the dark area fixed and open only the production-part. But with overlapping of at least 10 cm

Can I use a Car radiator? Because I have one, about 23cm of diameter
-> May be too big to fit to the front and they are sometimes very strong, may be produce too much air-flow. 

make it work by electricity with transformer instead of Solar Panel…
no problem! Just switch it off manually at the evening. There a small transformers from China where you can switch the voltage 7,5V, 9V, 12V. In this way you can modify the speed and adjust it better.