If the goal is not to waste money, then immediately throw out all ideas of changing the exhaust. It's been shown that no aftermarket exhaust improves the performance of the Fiesta ST. The stock exhaust is already optimized for performance. The only thing you gain with aftermarket exhaust is more noise.
If better performance is your goal, you also have to ask better performance for what reason? If you wish to compete in sanctioned events, be it autocross, HDPE, or whatever, you'll need to abide by class rules to avoid ending up in a category in which the Fiesta ST is uncompetitive.
If you have no particular reason to want to modify the car you might consider just not doing it. There are few directions you can go that won't end up in the big bucks category very quickly. The Fiesta ST came stock equipped with some of the best tires available--they were TW 140! So to maintain best handling you need to continue to get top quality summer tires. The RE050A is no longer available. If you are staying with the OEM 17" wheels, there are only a few choices that are even comparable. If budget is key, you would probably want Yokohama Advan Fleva V701 or Firestone Indy 500's or other UHP tires like the Kumho Ecsta Sport or BFG G-Force Phenom T/A. There's only one Max Performance Summer, the Michelin PS5. If you switch to a 16x7 wheel you can get the top street performance tire generally available, the Continental ECS02. These are all high performing street tires. You can go down the rabbit hole of track oriented tires, but these highly compromise normal street driving, particularly in terms of durability and wet performance.
In terms of increasing engine power, you can fairly inexpensively gain a little power with not much detriment by getting a Cobb Accessport and getting a tune. Some tuners are better than others and you can damage the engine or run afoul of the law with tunes so YMMV. Once you start tuning, you may find that the cooling capacity of the radiator isn't sufficient (though in Michigan maybe not), so if you do see higher temperatures then a radiator swap can be in order. Also as you go that route you may find that you want more intercooler capacity so that's a common swap. After that there are turbo changes, either a hybrid turbo or turbo swap. You can take the engine to as much as 300 hp but it will cost you and it most likely will give a shortened life to the engine.
Brakes as they come from Ford are as good as you'll need for fast road driving. Make sure you're getting the right performance pads as Ford has two grades. Brake maintenance is important, including regular fluid changes at 30,000 miles or more often. This is even more important for the longevity of the clutch slave cylinder. Track driving will result in heat problems with the brakes so upgrades there may be warranted or even inventing ductwork to bring more air to them.
Suspension-wise, it's hard to get better, safer handling than what the car comes with. You can get aftermarket coilovers and lower the car but that's going to change the appearance more than it improves any handling or ride. Likewise you can change swaybars and alter some characteristics of the handling. But you do have to be careful because some combinations of suspension modification can increase the possibility of rollover.
