If you go to the dealership you will pay 700$ per wheels plus 400$ per tires. That's a lot. Check out some sellers on here in the wheels and tires section you will find some replicas that are way cheaper and will fit your needs.
As for the speedo, nothing to worry if you stay within the OEM specs (245/40R20 if you go with the square setup which I recommend if you want to rotate the tires to even the wear). As for the wheel size, I am not 100% sure but I think they are 20x8.5+25 and the lug pattern is 5x120. That would be the RS square setup.
This website has plenty of info.
https://www.wheel-size.com/size/chevrolet/camaro/2018/
As I said, do not hesitate to PM the sponsors on here if you have questions you will get a better deal than with the dealership.
Good luck!