Personally, Id say drive it stock for 1000-2000 miles before you start to do performance mods to get USED to how it handles etc, its fun stock, you will feel confident, a main reason people love the FiST's!!! Get a $20-40 Gas Pedal spacer and LEARN how to HEEL/TOE! The pedal spacer raised the gas pedal by about 1" which makes Heel/Toe sooo much easier, I wear a size 10 Shoe... you'll thank me, trust me!
I wouldn't do much to the exterior, parts are Limited and too expensive in my personal opinion but... I'd get RAIN VISORS, 20% tinted windows, diffuser fins and maybe Rally Flaps, All mods I had installed...
- Id highly recommend getting some HID LED's for the LOW beams off Amazon, They run about $50 and are SOOO WORTH IT! I had 10,000k Lumen Low beans & 7k lumen fog lights, shit was stupid bright at night hahaha
Check out DIZZY tuning Mod path. Basically everything one can do to a Fiesta ST!
https://dizzytuning.com/blogs/technical-documents/the-ultimate-guide-to-modding-your-fiesta-st
Regarding interior AND Underglow Kits, I ran OPT7 (got off Amazon) and had their interior and underflow kit on my ST for I wanna say 3-4 years, ZERO ISSUES! They cost about $70/each and were so awesome hahah Fast and Furious Tokyo vibes for sure!
Other than that, Id HIGHLY recommend getting a Cold Air Intake, Rear Motor Mount, Symposer Delete - move plastic 1" diameter hose under the Intake filter for more cabin fun noises, INTERCOOLER & COBB ACCESSPORT and a tune from DIZZY, Tune+ or some other reputable tuner! Catted Downpipes aren't really needed as the stock is fine for now, some people even keep the stock DP and run the WHOOSH HYBRID Turbo for reference, Id put that $400 DP money towards the Intercooler, CAI or Accessport, not even a question tbh...
I wound up getting mods in this order and was pretty happy with my FiST..
CAI, RMM, Intercooler, Accessport w/ custom 91/E30 tune from Dizzy, Flowmaster CBE, Symposer Delete, and then a Turbomart Plumb BOV since the stock over time had slight issues holding 25PSI consistently and then I forget but that's the gist! Regarding the RMM install, its just as easy as changing your oil, seriously.... Have fun!
Sadly my FiST was STOLEN while I was attending a baseball game 3 nights ago... Im so upset, I LOVE my FiST and YOUR GONNA LOVE YOURS TOO, if you enjoy driving spiritedly!