Get a G-Body Buick (Or Olds, Chevy, Pontiac). Find a base model or one with a blown engine and buy the car for $500 or get it for free (like mine).
Strip it down and build up a new V8 (or drop in LT1 or LSx), manual trans, beef up the rear axle, upgrade the front brakes, and finally the suspension on one of these is just about skies the limit now, you can buy complete Pro Touring style kits for the suspension these days that can really help you tear things up on the autocross tracks.
Oh and remember to stiffen the chassis with factory parts (found on various models) or aftermarket copies (frame ties front and rear, various cross bracing, under hood bracing). Oh and to add a bit of lightness to the mix dig up the various factory aluminum parts.
