Oreca didn´t homologate any bodywork for Rally2+Kit, only things under the bodywork. So you can use what car brand/model you want.
Biggest problem of Rally2+Kit is price, people are still driving with Impreza/Lancer because of that.
With some brands you could self built Rally4 (R2) car, I expect that similar will happen with Rally3, you don´t have to buy "ready to drive" car.