Looks like a fun project.
Building a car for rallycross is a 1st step, minimal safety requirements and inspections. A fun and inexpensive way to test the sport and see if you really like and network with stage rally drivers/events. google scca rallycross and your state/region for local events.
Building a car for stage rally is a real challenge, lots of $$$ and strict safety and inspection requirements. Are you knowledgeable about those requirements?
And lots of money and logistics to enter and run in a stage rally event.
Many buy an already built and logbooked rally car, far less expensive. Here is a great source:
i've RX'd for 10 years and volunteer at many stage rally's so know much about all the above.