happy to help and congratulations
No flexfuel kit is not necessary, but a good add on to get the most out of a stage 2 setup and the peace of mind when having to run different qualities of fuel as well as ethanol. ethanol is cheaper, your engine runs cooler and smoother (the "smoother" bit is more of a personal observation)
You can add it later on as you get bored with just stage 2 with 93. Now I cannot NOT put ethanol as the car feels slow on 93
I think your plan to do this gradually is spot on as one more benefit of this would be multiple AHA moments. Eventually you get used to any amount of power and always looking for the next step. Might as well be more granular and enjoy the journey.
OEM coils are superb and no need to change it uptil 900ish hp, unless you run into issues which is rare. Even then replacing them with a new set of OEM ones would be the way to go.
The OTS maps from MHD are pretty robust, safe and performant.
for spark plugs too, once you go custom tuning your tuner would be best person to ask, until then stock plugs are fine. (them waring out due to high milage might be another thing but for OTS maps they should be fine.)
For tires, my pick is Michelin PS4s summer tires but unless your tires are worn out or you are concerned about getting the best of a lap time on a circuit you should be fine for a light spirited driving.
Also
https://www.youtube.com/@kern417/videos
This is my goto place for info on all things B58. Great guy, awesome content.
EDIT:
I have recently become a fan of this channel as some of their B58 related videos provide a rare insight which I have not encountered anywhere else.
https://www.youtube.com/@HarmonixxTuning/videos
Particularly these 3 videos
super informative.