Honestly they are incredibly easy to take care of. Heating can be left upto your house heater as they are fine with room temp, just make certain they have one spot in the tank with a heat lamp so they can heat up more if they want to. Their diet is easy too, you can go to almost any chain store and just pick up a thing of their gecko food, it'll cover everything they need and you can also feed some calcium dusted crickets as treats. I would avoid using gravel in their tank as they could accidently eat it and get bound up, personally I would just use papper towels as it a very easy clean up and they shouldn't get bound up if they eat some. They really don't need climbing space as they are mostly a ground bases lizard, granted a few big rocks wouldn't be a bad idea for something to climb on, mostly for entertainment. As for tank size they usually do just fine in a 20g long so a 75g is like a mansion to them, granted that's just speaking of a single gecko. Also give it a light misting once every week or so, just helps to maintain internal moister, lastly the fatter the tail it's got the healthier it is.
Personally I would scape the tank like a semi-desert type biotope. Just a few rocks here and there maybe some fake cactus like plants, a couple of places to hide from the light if they want and a heat lamp over one of the rocks for basking. You could add a couple sticks in but nothing that travels upwards in the tanks as leopard geckos aren't the best climbers and can be a bit clumsy lol.