Hi Richard,
If you’re committed to buying an iMac, this is worth reading … https://photographylife.com/how-to-buy-imac-for-photography
If you’re open to a PC, I'd read this … https://photographylife.com/pc-vs-mac-for-photography
Personally, I use a 5-year old Lenovo W520 laptop that works great. I've upgraded it to 16GB of RAM and installed 2 fast SSDs, which helps a lot with image processing.
Happy shopping!
Mark