As long as the offset, bolt pcd, center bore size and wheel width is correct your half way there.
Some 18" wheels require a steering limiter to stop the wheel rubbing with in the arch.
But saying that I know a few guys that are running 18's with no limiters and no problems.
Also be careful even if you have all the correct dims, depending on your brake set up, the wheels still may not fit or need it may need spacers so just be careful
If you fit spacer bare in mind you may need longer wheel bolts to
Have a look at this link from the FAQ section it my be helpful
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