Disclaimer: Nothing below is official, it is just what I've gathered reading the forums and entering races. It is possible that the organizers dislike everything I'm about to suggest.
To get an idea of how difficult it will be to get into a given race, you can look at the entry forms list for that race (go to the "team select" page) and count them. Usually races have about 100 cars, so once there are more than 100 entries, you need to really start working on your entry.
Assuming you're planning on entering Thunderhill, you probably will need a pretty decent theme. There's a big base of well-decorated teams in California, so getting into Thunderhill seems to get more difficult each year. I think [vaguely remember] there were 160 entries for 100 grid spaces last year.
Ideas for an entry without a car:
-Pictures of mockups, sketches, etc
-Lots of specificity: make it looks like you've got it very well planned, you just don't want to spend money without getting a grid spot first
-Have your roster filled out so it looks like you've got a committed team and will likely make the race
All that said, both races I've entered we've actually purchased and decorated the car and started installing the rollcage by the time the accept/reject deadline passes.
Car to Pit telemetry (OBD2, GPS, and analog inputs) with little more than a phone, router, and laptop. It's not MacGuyver, it's WifiLapper