What class will you be running in? What sanctioning body...
The S4 we ran/run is all stock S4 rebuilt motor. The body is not seam welded, with a 6 point cage and regulation door bars and it's been fine for years of rally and rally-x abuse.
The best advice I can offer is SKID PLATES. The rx7s have a small amount of travel in the suspension and once you load the car up with the weight of the safety equipment and driver/codriver you start to bottom out quick.
Get a taller spring if you can (we run vert springs, not sure what options your FB will have)... run a full skid under the engine/trans and again under the fuel/rear end. It adds more weight, but it has saved us more than once from certain disaster.
Also - get another parts car... if you do destroy your car during a stage, you can at least come back at the next event
