Welcome! It's nice to see you are researching care BEFORE you acquire your corn snake. You didn't specify the age of the animal you are getting. I mention that because it might have some bearing on sizes of everything, including overall habitat. Hatchlings are easily overwhelmed. So large enclosures sometimes make feeding difficult. And hide size should be matched to the animal size as well. Snakes love snug hiding spots; think cats with cardboard boxes. lol More than one hide is nice to offer as well. One on the cool side and one on the warm. Pick something simple that is easily cleaned. Old clay plant pots turned on their sides or upside down with a crack for an opening work well. Try to avoid those beautiful resin tree sculptures sold in the big box pet stores. The internet is rife with horror stories of animals getting jammed up inside the tiniest opening and then either being found dead or in need of rescue. The water bowl should be big enough for the snake get into and submerge themselves. Some corns like to lounge in their bowls (be aware it's a common sign of mite infestation when an animal suddenly begins spending their days in the water, not to be confused with soaking before a shed which is normal), others only drink and the PIAs poop in them! Pick something they can't tip over as well. Changing all the bedding several times a week will wear thin quickly. Given that corn snakes are basically nocturnal, lighting isn't strictly necessary (there are scientific studies that suggest access to UVA and UVB has some effect on health but nothing conclusive that I have seen). So lighting for your terrarium should suit your needs more than address the needs of your corn. Keep in mind that any lighting you choose should not add to the ambient temperature of the tank. Also when providing artificial lighting photoperiod matters. So use of a timer to turn the lights on and off helps keep the light cycle regular but not required. On to heating. I have had great success with belly heat. Under tank heating is easy to use and regulate with a herpstat. DO NOT stick any heat source to the bottom of your tank; even though the manufacturer recommends it. Doing so makes it impossibly difficult to deep clean and if you want to upgrade/replace enclosures removal of the heating element risks damage that can, at worse, cause a fire. Rather attach the heat to a ceramic tile and place it at one end of the tank, opposite where the water bowl has been placed. Lastly I recommend that you buy a digital thermomator/s for the tank so you can be assured that the temps are in a good range. This will help you eliminate potential issues with feeding and digestion.
I hope this blather has helped a little. It would be nice if you come back and share with us your new addition.
Good Luck,
Terri