I am looking for something like the Original Gran Turismo set up.
Real cars that you build up as you win races.
No jumps, no rocket boosters, nothing like that.

Dirt 4 is incredibly real - and difficult. I went out and bought a driving chair.

Gran Turismo was the benchmark but Forza Motorsport surpassed it a few generations ago. And now Project Cars is what I consider a top tier racing game. It's not a simulator but none of them are and it's extremely realistic. And because it doesn't try to include every make and model on the planet--something Forza goes out of its way to do--it offers an experience using the cars most dedicated to actual racing. It even includes dirt track racing, though Dirt 4 is probably going to be the better choice if that's your only desire.

Dirt sounds like the Colin McRae games I had on the original XBOX I think. Too complicated.
I grabbed Project Cars for $8 to give it a try.
Which Forza is the best???

I am just looking for something fairly simple. I want to buy a car as I race and win get to modify it or trade for something else, build a little garage with a few different cars to race just to mix it up.
I don’t get much time to play. I may get to do a few races a week.

In that case, maybe something from the Forza Horizon series would be better for you. It's a more arcade-like version of Motorsport, offers a wide variety of cars and racing styles (street, track, and dirt), and has vehicle customization. One of the things you might or might not like involves managing the in-game community and the Horizon Festival around which the game revolves. But, again, it's a casual version of Forza which you might be looking for.

But if you're dead set on "most realistic," then get the latest Motorsport or Project Cars.