There are lots of ways to improve the suspension. It really depends on your goals for the car. A kit from the right aftermarket company can be a good option, but those kits can also be really expensive. And, in some cases, overkill for a street car. I will never tell anyone not to build the car they want to build. But I do think it's helpful to let people know that it doesn't take many mods to make these cars handle much better than stock. And some popular mods will make the car handle worse. Here are some basic guidelines that I give to people who are setting up their Mustangs for autocross:
1. Avoid going too low or too stiff. I don't like to lower my SN95 Mustangs more than 1". Ford Performance C springs are my preferred SRA option. Most aftermarket "performance" non-adjustable shock & strut kits are stiffer than stock. A good set of adjustable shocks & struts allows you to dial-in the suspension (it's easier to do than most people think).
2. Stiffer sway bars (especially a stiffer front sway bar) aren't always better. I posted a detailed video about this on my YT channel. For a V6 with an OEM front sway bar, look into a 23 mm OEM rear sway bar. When you swap to the V8, an OEM GT front sway bar with a 25 mm aftermarket rear sway bar is generally a good start. A stiffer front sway bar is the last mod I make to my autocross cars.
3. Better bushings make a big difference. Simply putting new rubber bushings in the car will make it better. But upgrading to poly/Delrin/spherical bushings can be a relatively inexpensive way to improve performance. There are lots of options, and I recommend different ones for different budgets/goals. If you'd like more details, let know.
4. A square wheel & tire setup is better than a staggered setup. Staggered setups are very popular, but they add understeer to a car that already has a tendency to understeer. If you plan to use the car for performance driving, a square setup is better.