Hi mate, for rota grids they come in the perfect sizes and offsets which are 9.5 front, 10.5 rear. ET20. This will clear the brembos and give a great looking stance without the need to start messing with the arches. As for those second hand ones, essentially just space them to ET20 and they will be the same.
Speak to Chris at Tarmac Sportz if you after a new set
(just seen you are after 18" not 19" so not sure if they do the sizes i've mentioned in that size but i dont see why they wouldn't and as for the widths and offset, they will be the same anyway)