Gravel and sand

Starting a new tank...haven’t had one in many years. Want to use gravel and sand, as we plan on some Cory’s, some loaches, and maybe a pleco or two. The tank is 75 gal. I’m trying to get an idea of how much sand and gravel to use. The tank is not tall, and we may include some live plants. I want sand because I know the Cory’s and loaches like to dig around a bit. We won’t be putting any of them in for awhile because we need to set up and get the tank all cycled out yet. Any advice will be welcome...thanks!