If you're gonna go up to 93 octane and past stage 1, it's recommended you also upgrade the intercooler, radiator(triple pass), plugs, and exhaust. The radiator, imo, is the first thing you should do if your engine suffers from heat soak.
I was down in Lone Pine, Ca this last summer. Temps above 100F. I couldn't climb the road to Whitney portal because the fiesta kept overheating. This was on a stock tune. It happened in Tahoe too. Very annoying. The radiator upgrade also netted me a ~3mpg improvement. This also wound up taking out my bypass solenoid which I had to replace in a napa parking lot in 110F heat with a half skin-degloved hand, lol. Super annoying.
So go ahead and do the plugs. A lot of folks recommend a step colder. Make sure to gap them well.
After that you're hitting the limits of the small lil turbo we have. Then you'd start upgrading the turbo, fuel system, etc. After that you'll have to start looking at an upgraded block.
For parts, keep an eye on the for sale section here on the forums. It's pretty active and lots of folks are parting out their fiestas lately. Facebook marketplace is also solid. Ebay is really hit or miss for me on these cars but ymmv.
Welcome to the club! Have fun!