Hey Sam,
These wheels should be fine for an R33 Gtst, being an 18x8 35+ and 18x9 38+ they should sit very close to where the standard rims would.
If the rears were 40+ they would be better but being 38+ just means that they will sit further out by 2mm (not much really)
I dont see there being an issue with clearance of either guards, brakes or inner guard covers.
In saying that I never recommend buying wheels that you arent able to test fit first, but that may be a bit difficult in this instance.
You will need to do a bit of research on the best tyre fitment though (I can help you there).
If it were me, I would buy the Volk's not the cheaper imitations.