hey mate, sorry didnt see this before haha, +35 with a 18x9 is cutting it very close on the front upper control arm, ive run quite a few 17x9 +30's on the front and theres only been a few mm of clearance. 18's will come up even closer once again, if you look at the angle of the control arm and how it sits ull see what i mean. also tyre size will play a big roll in this, obviously a bigger tyre u have less clearance, if it comes down to just the tyre fouling and not the rim, then maybe try something around a 225/40. if you are dead set on the wheels and dont mind running spacers then id grab them and make up mock spacers to see what size u need, then go buy a pair, ull probably only need some slip on 5mm's or something which cost shit all.
as far as the rear is concerned, once again ull be cutting it very close, but unlike the front, having a larger diametre wheel actually gives u more clearance funnily enough. if you look at the back ull see (depending on whether ur car has HICAS or not) u have a rear tie rod end for the HICAS which sits there, a 17" wheel around the 10" around the +35ish mark u will be coming bloody close if not already fouling on it. however an 18" sometimes will actually just clear this and u wont have any fouling issues there. ive heard of some people having this issue and some people not so id imagine it has something to do with the design of the barrel/rim as well as the diametre.
i wouldnt be worrying about the rears as much as if indeed the wheels didnt clear over the HICAS end, then it would be a matter of a spacer of maybe only a few mm thickness.
i cant remember, but i beleive the stock R34 rims are 17x7? yes u can go 235/45, as the rim has a fairly high offset im sure uve already realised that it sits in the guard a fair way, so there shouldnt be any fouling issues on the guard, and since going from a 225 to a 235 u should only expect the tyres to bulge out by 5mm each side (235-225=10mm, 10mm/2=5mm) so there shouldnt be any fouling issues on the inside either.