Best book on the market is Shep Paines, How to build diorama's. While geared more toward military items it is full of hints and tips on groundwork etc.
As for your idea, depends on what you want to show. Do you just want a showcase to show off your latest builds or do you want a more permanent display? If showing off your cars, you can make either a simple parking lot or a roadway. For a more permanent display, you need to come up with a story line (that is the point of a dio, to tell a story). Lots of ways you can go here. Police pulling over a speeder, someone working on their car with it on jacks and the hood open to show off the engine detail. Pit scene for racers, junkyard scene, gas station, car show, etc.
You can purchase lots of accesories for cars in 24th/25th scale including people.