Before anyone gets to answering my question I did find this:
http://www.porsche964.co.uk/technical/964%20wheels%20and%20tyres.pdf
Based on what I am reading I'm going to stick with 16" or 17" rims. Looks like you can make 18" work but I would rather stick with what's considered normal.
I know there are a ton of wheel question threads already but many are old and the ability to get various wheel types changes over the years. I've checked tire rack and the choices appear to be minimal. Are there other good sources I should know about?