Basically, these aftermarket firms are pushing you to buy a brand new suspension just to replace your worn-out service parts. New bushings, ball joints, tie-rod ends and shocks, to factory HD standards, would make a world of difference. HD torsion bars and rear springs, and any of the available front/rear antisway bar setups (even the cheapie Addco) would basically give you new car handling, if combined with updated alignment specs and quality radial tires.
Be real -- are you prepping to run the Silver State Challenge? Do you regularly autocross or road race? Don't get conned into a full-on Pro Touring conversion unless that's what you have your heart set on. If so, be prepared to spend money.