OK, so here is some homework for you to do.
1. Other than the engine harness, what other things do you want to keep? Headlights, tail lights, windshield wipers, turn signals, mirror adjustments, power seat, radio
other things. I assume things like the brake lights (required) and the fuel pump (also required if you want to actually get the car onto the track).
2. What gauges do you want? What gauges do you actually need?
As for being easy and cheap to build a race car, it is, other than the cage. The issue is how far you want to go with car prep verses driver prep. As Charlie is fond of saying, get a basic race car and spend time and money on the driver to move up into the middle of the grid. Later, when/if the driver gets better and is ready (and wants) to move towards the pointy end of the grid, continue car prep to get a "no excuses" competitive car. A "basic" race car will not be good enough to get a good driver to the front against other good drivers with well prepared, well tested cars. That is just a fact of life in any form or racing.
So, set your goal for your racing first and then develop your build plan. Do you want to contend for the win or for third? There is a BIG difference. If you want to contend for the win, forget worrying about car show stuff and develop your car with that in mind. I, for one, do not believe that having a developed car will turn a new guy off. I think most will understand that getting out there is, relatively, cheap and easy. I think what is more important is that your car look clean and neat. That is what will attract guys. When they show interest, the different levels of car prep can be explained. So, do you want to contend for the win or for third??????
Big Dog