RE: beat it
Definitely get a car that you won't wince every time you hit a rock or bounce off the landscaping. Realize that, sooner or later, you are going to wrap the car around an immovable object; if you can afford to throw it away, you can afford to rally it.
Also definitely work rallies while you either build your car or save up to buy one already built. Our first car was a used Toyota Corolla, and it was a great car.

Engine like a mule!

We built our Nissans and Mazdas, and bought the Geo mostly built (suspension, a new skidplate, and harnesses are the only changes so far); building is expensive, and NEVER SKIMP ON THE SAFETY!!!
Get your cage built by someone who knows what they are doing. Make sure, if it's not someone who has built rally cages before, that they understand exactly what you will be doing with the car. Show them video of Crash Kings of Rallying, so they understand that bad things happen and you'd like to survive.
Get the rule book, memorize it. Get the rule book to your cage builder. Make sure your co-driver has at least read the rule book once.
And above all, work rallies!! There is absolutely no better way to figure out how to rally than to work. Work timing controls, because then you know how to do it when you compete. Work ATC, Start, Flying Finish, and FTC. Work road marshall so you can see what people do to their cars.

And, as Brian says, then you can see how it feels to be abused by some of the "best" teams in the country, and when you compete, you will be nice to workers.

Have fun, welcome to the craziness!
Kristen