Ordered new AFR sensors for the BPE from Rock Auto in the USA last Monday, just checked the tracking to see where they are...for some reason DHL think the most direct route to from Los Angles to Auckland is via Slough in Great Britain...
I would say that 18*8 +45 would work no problem. With a +35 I would say that it may poke beyond the rear guard could perhaps fit with some work. (24.4mm more poke than v7 STI wheel)
18*8 +45 vs 17*7.5 +53
http://www.willtheyfit.com/index.php?width=225&aspect=45&diameter=17&wheelwidth=7.5&offset=53&width2=225&aspect2=40&wheel_size=18&wheel_width=8&offset2=45
When compared with v7 sti wheel which fits for me being 10mm wider on either side. As long as you are prepared to roll the rear guard I would say anything up to 8.5 wide with appropriate offset would be fine.