Hello! Fellow 3rd gen newb here.
Basic question but I can’t find an answer anywhere. I am wanting to convert my 2015 IS250 interior lights to LED. My curiosity is if I replace the bulbs will the fade in/out courtesy lights on the map and floor still fade or will that stop.

 

It will work but not as smooth. You'll probably get flicker at lower light levels and won't go as dim before turning off completely.

 

I replaced all of mine and there are a least 3 threads on this topic in the 3IS section.
There are no flickering issues that I experience [but is possible with LED] and they are a huge visual improvement as they are a higher luminescence.

Cabin:
How many you need (if I am not forgetting any):
1x trunk
10x cabin (1x for each front foot well, 1x for each vanity mirror, 3x front ceiling, 3x rear ceiling)
Optional: 2x exterior side mirrors (one each), there is a youtube tutorial on how to take them apart to replace the bulbs.

Doors:
F sport light
1x for each front door (comes in a 2 pack), (if you want the F Sport two colour one it is on ebay)

 

One thing I've done in every vehicle I've owned with a trunk is wire-tap into the trunk light with two 12" Autozone LED strips and stick them to the bottom of the rear deck. You can't see the light source but my trunk lights up nicely. Due to my job, I'm digging in my trunk at night quite often and it looks OEM.