You're gonna have to shop for what's available.
A few years back, I had about 2000 ft2 shipped to me from here:
Was a trouble-free experience. Zero breakage in transit.
Tiles were 24"x24" 9mm thickness.
Find a color scheme and size/shape you're interested in, and then go from there. You're going to find more PEI4's than 5s. From the samples I picked up, I couldn't find a significant difference between them. The important things are going to be whether it's a through body porcelain or printed. Rectified or not, if you really care about grout lines and lippage. And price.
I'm an amateur tile setter. Only done about a dozen projects. IMO I don't think thickness is going to matter if the substrate is solid and the installation is done in accordance with the thinset's guidelines in terms of coverage.
Is this a DIY or pro job?