Probably easier and least expensive would be to buy a rally ready car. Unless you got skills, time, tools and motivation to do most of the work yourself I wouldn't build it. These are minimum prices, so it doesn't mean it's the best option:
-roll cage can't really buy a good kit, needs to be build in the shop $1500 or DIY $500
-seat belts $250
-seats $500
-skid plate $150
-helmet $250, suit $250, hans $600
-intercom, rally computer, suspension, wheels, tires, lights, various bits and pieces, ... $xxx-$xxxxx
And then the real expenses start when you begin attending the events..