Maxing out with full compatibility for Windows 7 would be the Z370 chipset, good boards can handle the power requirements of the top spec i9-9900. Although for games a i7-9700k is near identical and hyper-threading usually harms minimum frame times so you may end up disabling that on the i9 which means the i7 is much better value if you're not using productivity software. Some of the latest games suffer from stutter on the 6 core non-hyperthreaded i5 as they're using code from 8 core consoles. Lower end 9th gen CPUs don't get the soldered TIM between the silicon and heatspreader.
The Intel 900p PCIe SSD would represent the best possible storage with crazy random performance but Samsung M.2 drives are still great.
Current Nvidia cards are still getting Windows 7 drivers, maybe the best Windows 7 GPU is yet to be released. The previous gen Nvidia cards are very close in performance to the current ones and allow a wider choice of drivers. Anyone who bought a 1080ti on the cheap from someone with an RTX preorder should be feeling happy.
This stuff is so recent that it's current and expensive so now isn't the best time for prices, high spec DDR4 will come down in price for sure. However, good motherboards hold their value to it might be an idea to pick up a new Asus ROG z370 (not Strix) motherboard while stocks are being cleared, rather than spend the same money on a used one later that might have been abused.
Edit: I started writing this before your post about AMD only, although doesn't change what is the absolute maximum for Windows 7 before major compatibility headaches.
Screw period correct; I wanted a faster system back then. I choose no dropped frames, super fast loading, fully compatible and quiet operation.