What sort of tasks will your client be doing? "Big and flat" projects (high resolution photos with retouching, maybe a few layers) or "tall and thin" projects (things like web graphics, or lower resolution images but that have lots of layers and visual effects applied)? The architecture of computer you want to build will depend a lot what kind of files your client is working with.
For someone who is a photographer for example the RAW photos from a day worth of shooting can easily be over 40GB, there you greatly benefit from having larger capacity SSD and will see a differene between high performance SSDs like a M.2 drive and low cost SSDs. You also benefit from setting aside a partition on your SSD to use as a scratch disk (virtual memory). That way if your client is doing something like photomerge or content-aware fill on high megapixel photos and runs out of memory, he won't be waiting a couple of minutes for your 7200RPM hard drive to write several GB of memory contents to a drive.
If your client is not working with large sets of photos or very large PSD files, I'd get a small SSD just enough to hold the OS, Photoshop, and a few projects.
For "tall and thin" projects (lots of visual effects) you'll mostly be limited by CPU.
Note that for while Photoshop does utilize additional cores if they are available some filters are inherently serial and cannot be split into multiple threads so having good single thread performance is important. Go for the Intel chip.
GPU is not that important for Photoshop unless your client is specifically using 3D photo effects functionality. Just about any discrete graphics card will help speed things up like panning and zooming, image resizing and a few other things. This is something you client could pick up at a later time if he finds a deal or something. CPU, RAM and SSD are all much more important.
I'd go for 16GB of RAM and if possible get a motherboard with more slots (e.g. 8 DIMM for a 4-channel memory system) so your client can add additional memory in parallel if needed in the future as needed, or if a good deal becomes available.