Do you want to build an actual rally car? Most of the ones I've seen at Rally America events are Imprezas or other economy hatchbacks like the Ford Focus and VW Golf, although once I saw a Porsche 944. Per regulation the cars have a full interior roll cage and fog lights, as well as off-road tires. Usually people have beefed up bumpers and/or performance exhaust as well. On a more personal note, my friend and I are building a rally car out of a 1985 VW Golf 4-door with a VR6 out of a 2000 GTI. It's not too fancy right now, just a baby blue hatchback with white doors.