I've done loads of research on these specific RPF1 wheels.
If you want to clear UK/TT brakes without requiring spacers and longer wheel studs also avoiding any arch work etc then you want to go for 17/18x8.5 et30 at the front and 17/18x10 et38 at the rear.
I've just ordered a set of RPF1 18x8.5 et30 front and 18x10 et38 rear.
As for tyres I'm going 245/40/18 on the fronts and 275/35/18 rear. This should have no stretch as I had the same setup on my previous wheels which were 18x9 fronts and 18x10 rears.
17s look too small on a MK4 Supra to me personally, 18s will fill the arches nic