Welcome along
With regards to mods:
Not all bucket seats fit, make sure you try before you buy in terms of comfort, and brackets may need to be custom jobs
Harness bars are def available
Harnesses likewise
JWT Popcharger would be my choice as it's cost effective and whatever you do with a cone filter you'll lose power, worth it for the noise though
Don't get anything from Crapspeed, buy some decent Berk ones
Y-pipes don't usually have silencers on, do they? So no need to go custom
Steering wheel easy to do
Springs are easy
Tyres go for MPS4 on 18" (and well done for function > form), Rays are fine and cheap and light as you'll get before ££££
You'll def want spacers for the Rays