Rattling can be a symptom of many different issues. Worn out bushings, damaged suspension components, bottoming out the suspension on the bump stops (which happens a lot with the stock torsion bars, especially if the car is lower than stock). And of course just being an old uni-body with no reinforcement.
A set of torque boxes and subframe connectors will make a big difference. There's more out there that you can add, but that's a great start and those two by themselves probably make the biggest difference.
Coil-overs do not improve handling in and of themselves. That's a myth. There's nothing magical about them, they're just springs and shocks. Just like a torsion bar and a shock is a spring and a shock. People say they improved their handling because usually they're replacing old worn out factory torsion bar suspensions, they're not comparing their brand new coil-overs against a brand new rebuilt torsion bar suspension with modern components. If anything, the fact that these cars were not designed with coil-overs in mind places geometry and travel limitations on the coil over conversions that don't happen with the torsion bars.
A rear sway bar being needed depends on the rest of your suspension set up. From the factory the front suspension was more undersprung than the rear, so rear bars weren't really a benefit, they tended to make the car oversteer. However, if you upgrade your front suspension, you may find a rear sway bar comes in handy in keeping the suspension balanced.
Not sure what disk brakes you're running now, but Wilwood is just overpriced eye-candy. Their 3" hub diameter limits your wheel choices substantially, and they're far more expensive than kits that perform just as well. For a street car I'd rather have a more factory style kit with easy to find parts.
